A novel photosensitive poly(amide-co-imide) was synthesized from the copolymerization of p-phenylenediacryloyl chloride, bis(3,4-dicarboxyliphenyl)hexafluoropropane dianhydride and 4,4'oxydipehnyldiamine followed by subsequent chemical imidization. The resulting polymer showed intrinsic photosensitivity without additives, good processability in solution and high thermal stability up to 350 degreesC. From the refractive index measurement of the polymer by prism coupling method, it was found that the refractive index could be varied precisely with respect to the exposure time (from 1.6444 to 1.6230 for 2 mum thick film, TE mode, measured at 1.55 mum). Fabrication of passive wave guide was attempted by the photo-patterning process, i.e., photo-modulation of refractive index. Straight waveguide was easily fabricated and single mode near-field pattern was observed by using 1.55 mum near-IR light.