A new method for measuring multi-layer optical film parameters is proposed, which is based on observation of quasi-waveguide leaky modes. As an example, two liquid layers separated by a glass slide and sandwiched between a prism and another slide, were studied. Leaky mode mlines of the two layers were measured, and used for calculation of the parameters of the two layers. The errors of refractive index of the measured film and the thickness are about ±0.001 and ±0.1μm, respectively.