We investigated the stability of LB multilayers of three organic dyes, diazostilbenes (HPNA), phenylhydrazone (PD) and hemicyanine (DAEP), which all have large hyperpolarizability β but different double bonds in their conjugated π electron systems (N=N in HPNA, C=N in PD and C=C in DAEP). It was found that the second harmonic generation (SHG) intensities from those samples reduced significantly within a few tens of days. On the other hand, the Raman intensities from the powder samples of the three dyes due to the stretching modes of the double bonds and the UV vis absorption band from the LB multilayers of the three dyes corresponding to the original chromophores also decreased with increasing time of UV irradiation, confirming the breakdown (cleavage) of the double bonds (photo induced bleaching) in all the three dyes. The SHG, UV vis absorption and Raman experiments deduced the same sequence of stability: N=N the worst and C=C the best, suggesting that photo induced bleaching effect, instead of deterioration of the film frameworks, could possibly be responcible for the instability of the optical nonlinear LB films.