 <p>In order for your code to work, you have to reverse the logic i.e.</p> <pre><code>private void button1_Click(object sender, EventArgs e) { if(checkBoxWMVFile.Checked == false)//if the textbox is not checked { if (timercheckbox == null)//If this is the first time { timercheckbox = new Timer();//Create a timer timercheckbox.Interval = 10000; // Set the interval, before starting it. timercheckbox.Start();//Start it } else timercheckbox.Start();//If it is not the first time, just start it checkBoxWMVFile.Checked = true;//Check the checkbox } else//the checkbox is checked { timercheckbox.Stop();//Stop the timer checkBoxWMVFile.Checked = false;//Uncheck the checkbox } } </code></pre>