The photobleaching mechanism of nonlinear optical polymers has been investigated; the PMMA based polymers with nonlinear optical chromophores, 4-dimethylamino-4'-nitro-stilbene (DANS), 4-dimethylamino-4'-hexylsulfone-stilbene (DASS), and 4'-(2-hexyloxyethyl) ethyl amino-4-nitro azobenzene (DR-16). The photobleaching was carried out using near-UV lines. FTIR spectra and UV-VIS absorption spectra were obtained after each photobleaching. Refractive indices were measured in the photobleaching processes. The results suggest that the photobleaching mechanism of two PMMA based side-chain polymers with DANS and DASS chromophores results from the photo-oxidation and that of the PMMA based host-guest polymer with DR-16 chromophore results from the combination of the photo-degradation and the trans-cis conformational transformation.