user3458: Well, obviously if my opponent has only a King, I cannot lose...either a win or a draw. Suppose I also have a Queen(Forced win).
However suppose I want to lose (maybe I am winning and want to grant some points to my opponent or whatever), so can I resign under such a circumstance ?
(I understand that my opponent would have resigned before reaching such a position, but anyways is it legal to resign if opponent has only king? )

user18010: If you do that in a tournament, it could be construed as a conspiracy to help your opponents position in the standings, and you both would be disqualified.

user4418: Consider the following scenario:
The round is scheduled to start at 6:30pm and normally I would expect my game to finish comfortable before 11pm when I need to leave to catch the last bus home at 11:05pm. There is no zero tolerance. Instead players have one hour to arrive before they are defaulted. Normally I start my opponent's clock at 6:30pm on the dot if he hasn't arrived so that I have no problem with the last bus.
At 6:25pm the arbiter announces that he has had news of an accident on the motorway which is delaying some of the players. Consequently he rules that we may not start a player's clock until 7pm if they are not in the club. The default time of 7:30 is unchanged.
I complain bitterly to the arbiter but he is unmoved and won't change his decision.
My opponent eventually arrives with the other late players at 7:05pm. I started his clock on the dot of 7:00pm. The game drags on and on. Eventually at 11pm I queen my last pawn against my opponent's bare king. I offer a draw but he refuses. I knock my king over in disgust and head for the door and the last bus.
The next week the arbiter tells me that knocking my king over constitutes resigning. He also tells me that if I had walked out without resigning I would have "lost on time" but been awarded a draw because my opponent didn't have mating material.
This is perfectly legitimate. I have resigned against a bare king and this is legal. The only sanction I am likely to suffer is for not shaking hands after the game.
EDIT: This subject actually came up at the recent FIDE meeting in early September! Alex Holowczak (amongst others) was representing England at these meetings and his report appears here - http://www.englishchess.org.uk/wp-content/uploads/2015/10/C23.9.5.1-FIDE-Delegate-report.pdf .
Note, in particular this excerpt from Alex's report from the meeting of the Rules Commission:

A proposal noted that you could run out of time but not lose if your
  opponent had a bare King, and asked why you should lose if you resign
  but your opponent had a bare King. I commented that this situation was
  so unlikely/unreasonable that it shouldn’t be legislated for.


user1761: The FIDE laws of chess contain two parts: Basic rules of play (Sections 1-5) and Competition rules (Sections 6-12).
The first part is what defines how the pieces move, when the game ends, and so on. These rules indeed allow players to resign at any point before the end of the game, so you can resign right before you would give a checkmate, but not after you have released the piece after the final move. The second part explains things such as the chess clock, scoresheets, the role of the arbiter etc. Curiously this part seems to have nothing to say about deliberately losing. 
So technically, one could say that resigning in a won position on purpose "is allowed" by the rules of chess.
However, the FIDE tournament rules, which are used in international tournaments, say that

8.(f) Where it is clear games have been pre-arranged, the CA shall impose suitable penalties.

I don't have a good credible source, but I think that deliberately losing games (which has happened) are usually considered in a similar way. In addition, deliberately losing games is generally considered unacceptable behaviour in sports [citation needed]. So in my opinion, it is clear that deliberately losing a game is against the rules of a tournament (unless explicitly specified if the tournament organizers want to allow such tactical manoeuvres). You can always ask the tournament organizer if you want confirmation.

3458: <p>Well, obviously if my opponent has only a King, I cannot lose...either a win or a draw. Suppose I also have a Queen(Forced win).</p>

<p>However suppose I want to lose (maybe I am winning and want to grant some points to my opponent or whatever), so can I resign under such a circumstance ?</p>

<p>(I understand that my opponent would have resigned before reaching such a position, but anyways is it legal to resign if opponent has only king? )</p>
 18010: <p>If you do that in a tournament, it could be construed as a conspiracy to help your opponents position in the standings, and you both would be disqualified.</p>
 4418: <p>Consider the following scenario:</p>

<p>The round is scheduled to start at 6:30pm and normally I would expect my game to finish comfortable before 11pm when I need to leave to catch the last bus home at 11:05pm. There is no zero tolerance. Instead players have one hour to arrive before they are defaulted. Normally I start my opponent's clock at 6:30pm on the dot if he hasn't arrived so that I have no problem with the last bus.</p>

<p>At 6:25pm the arbiter announces that he has had news of an accident on the motorway which is delaying some of the players. Consequently he rules that we may not start a player's clock until 7pm if they are not in the club. The default time of 7:30 is unchanged.</p>

<p>I complain bitterly to the arbiter but he is unmoved and won't change his decision.</p>

<p>My opponent eventually arrives with the other late players at 7:05pm. I started his clock on the dot of 7:00pm. The game drags on and on. Eventually at 11pm I queen my last pawn against my opponent's bare king. I offer a draw but he refuses. I knock my king over in disgust and head for the door and the last bus.</p>

<p>The next week the arbiter tells me that knocking my king over constitutes resigning. He also tells me that if I had walked out without resigning I would have "lost on time" but been awarded a draw because my opponent didn't have mating material.</p>

<p>This is perfectly legitimate. I have resigned against a bare king and this is legal. The only sanction I am likely to suffer is for not shaking hands after the game.</p>

<p>EDIT: This subject actually came up at the recent FIDE meeting in early September! Alex Holowczak (amongst others) was representing England at these meetings and his report appears here - <a href="http://www.englishchess.org.uk/wp-content/uploads/2015/10/C23.9.5.1-FIDE-Delegate-report.pdf" rel="nofollow">http://www.englishchess.org.uk/wp-content/uploads/2015/10/C23.9.5.1-FIDE-Delegate-report.pdf</a> .</p>

<p>Note, in particular this excerpt from Alex's report from the meeting of the Rules Commission:</p>

<blockquote>
  <p>A proposal noted that you could run out of time but not lose if your
  opponent had a bare King, and asked why you should lose if you resign
  but your opponent had a bare King. I commented that this situation was
  so unlikely/unreasonable that it shouldn’t be legislated for.</p>
</blockquote>
 1761: <p>The <a href="https://www.fide.com/fide/handbook.html?id=171&amp;view=article">FIDE laws of chess</a> contain two parts: Basic rules of play (Sections 1-5) and Competition rules (Sections 6-12).</p>

<p>The first part is what defines how the pieces move, when the game ends, and so on. These rules indeed allow players to resign at any point before the end of the game, so you can resign right before you would give a checkmate, but not after you have released the piece after the final move. The second part explains things such as the chess clock, scoresheets, the role of the arbiter etc. Curiously this part seems to have nothing to say about deliberately losing. </p>

<p>So technically, one could say that resigning in a won position on purpose <strong>"is allowed" by the <em>rules of chess</em></strong>.</p>

<p>However, the <a href="https://www.fide.com/fide/handbook.html?id=20&amp;view=category">FIDE tournament rules</a>, which are used in international tournaments, say that</p>

<blockquote>
  <p>8.(f) Where it is clear games have been pre-arranged, the CA shall impose suitable penalties.</p>
</blockquote>

<p>I don't have a good credible source, but I think that deliberately losing games (which has happened) are usually considered in a similar way. In addition, deliberately losing games is generally considered unacceptable behaviour in sports <sup><em>[citation needed]</em></sup>. So in my opinion, it is clear that deliberately losing a game <strong>is against the <em>rules of a tournament</em></strong> (unless explicitly specified if the tournament organizers want to allow such tactical manoeuvres). You can always ask the tournament organizer if you want confirmation.</p>
