Abstract: An experimental setup was constructed to measure the thickness less than 0.5mm of the indissoluble liquid film.When the laser falls on the liquid film surface waves,steady and visible diffraction patterns were obtained.Based on laser diffration,the theory of dispersion relations for waves on liquid film surfaces was testified.Subsequently,the thickness of the liquid film was measured carefully.The result shows that the measured thickness corresponded with the actual thickness very wel1.Based on the principle of this experiment,a new method for thickness measurement of indissoluble liquid film is built in a real time and non-contact way.