Seven of the the Lib Dems' 57 MPs are women - numerically and proportionally the fewest of the three major Westminster parties.
All of the party's MPs are white.
Environment Secretary Ed Davey told the Guardian newspaper his party's record was "not good enough" and the lack of progress "deeply frustrating".
He was speaking at the end of the Lib Dem conference in Glasgow.
The Liberal Democrats have five ministers in the coalition cabinet but none are women.
MPs Jo Swinson, Lynne Featherstone and Jenny Willott are the only Lib Dem women to hold junior ministerial posts.
One of the party's highest profile female MPs, Sarah Teather, said recently she planned to stand down at the next election.
Ms Teather - an education minister until 2012 - was sacked to make room for David Laws, who had previously resigned over his expenses.
"Progress has not been good enough on getting more female and ethnic minority MPs. Everybody in the party finds it deeply frustrating," Mr Davey told the Guardian.
"Current efforts to get more female MPs without positive discrimination are rightly being tested to destruction. I would think long and hard about positive discrimination if it doesn't work next time."
The party has traditionally been averse to such methods, rejecting the idea at past conferences.
Leader Nick Clegg said in 2010 the Lib Dems were "in the last chance saloon" in terms of getting ethnic minority and female candidates selected to fight seats for the party.
The Lib Dems could introduce positive discrimination to redress the party's gender and ethnic imbalance, one of its cabinet ministers has said.