A method for measuring the frequency stability of tunable laser is proposed by using confocal Fabry-Perot (F-P) interferometer. The F-P interferometer is used to get the output frequency of the laser as a reference, and the method eliminates the need of an independent optical source as a frequency reference. Using this technique, the frequency stability of the tunable external-cavity diode laser (ECDL) is measured to be 2.26×10^?9 with an integration time of 20 ms.