Polymers that are suitable for thin films are also used as materials for hybrid optical IC's; however, it is not easy in general to get a high refractive index change in the waveguide using these materials. To remove this difficulty, we have used copolymers of glycidyl methacrylic acid and methyl methacrylic acid, doped with cinnamic acid; this acid was directly photolocked, by ultraviolet exposure, to the epoxy rings of glycidyl methacrylic acid. Eventually, a refractive index change as large as 2% was achieved. Single‐mode optical waveguides with various widths were fabricated by means of the present method, and the dependence of the loss on the waveguide width and the loss in a bend were measured.