Photosensitive properties of poly(dimethyl-co-methylphenylsilylene) copolymer(PDMPS) was investigated by the residual film method of measuring the change of solubility of PDMPS before and after exposure to light. The samples coated on quartz plates were exposed to light under various conditions and steeped in the same solvent as used for coating, and then the yield of residual film(W/W0) was measured. The yield of residual film was affected by the molecular weight of PDMPS copolymer, types of photosensitizer, and their concentration. The most effective photosensitizer for PDMPS copolymer among those used here was rosebengal. And also, the photocrosslinking mechanism of PDMPS copolymer was studied by infrared an d UV absorption spectra. The stability of PDMPS spinning fiber by exposing to light was investigated by the change of the solubility of spun fiber after exposing to light was investigated by the change of the solubility of spun fiber after exposing to light.