Based on plane wave theory, the propagation characteristics of TEo modes have been studied in a symmetric slab leaky waveguide of a nonlinear djelectric film. Our results show that steady TEo modes can not exist if the film thickness is above a critical value, two TEo modes with different peak field intensities may exist at most if the film thickness is below the critical value. With the decreasing of the film thickness, the peak field intensity difference between the two modes becomes large.