We report an observation of optical phase conjugate in LB films of pure purple membrane for the first time. The dependence of intensity of the optical conjugate wave upon the total incident intensity has been deduced, based on the simplified model of photo-cycle of bacteriorhodipsin [bR(570) M(410)]. The fitting restults of the saturated absorption intensity of the LB films and the life time of the intermediate M (410) are 0.49 W/cm2 and 2.5 ms, respectively.