We report a new kind of biomolecular LB films composed of a photosensitive protein-bacteriorhodopsin. The saturated absorption intensity and nonlinear refractive index of the material are estimated to be 0. 42 W/cm2and 5×10-22cm2/W and the typical response time of the nonlinearities is about 1ms. High to third-order self-diffractions by population gratings in the LB films have been observed in the degenerate two- beam interaction experiment. The relation between intensity of each order diffracted beam upon the total incident intensity is deduced based on a simplified photocircle model of bacteriorhodopsin. The saturated absorption intensity and nonlinear refractive index of the material are then calculated by fitting the experimental data with the theoritical formuls.