A new fabricating technique which is capable of easy pattern formation in gradient index polymer optical waveguides is proposed. Waveguides are obtained by diffusion and fixing of monomers with low refractive indices into a gel patterned by ultraviolet irradiation. The gradient index profiles can be selectively formed in the irradiated parts due to the difference in diffusion rate of monomers between regions in the same substrate. Waveguide optical properties are also demonstrated.