Photobleaching effect of an electro-optic polymer, a PMMA based side-chain polymer with NLO chromophore DANS, has been studied with photobleaching only, with poling only, and with both photobleaching and poling in that order or in the reverse order. The photobleaching was carried out using the near-UV lines. FTIR spectra and UV-VIS absorption spectra were obtained after each photobleaching. Refractive indices and electro-optic coefficients were measured in the photobleaching processes. The results suggest that the photobleaching effects result from the photo-oxidation of NLO polymer rather than the trans-cis conformational transformation of the NLO chromophores