The behavior of a closed co-extrusion process for the preparation of gradient-index polymer optical fibers is modeled theoretically. In particular, the effects of the essential parameters such as the volume ratios of the inner/outer layer, diffusivities of monomers, diffusion length, and monomer concentrations, on the distribution of refractive index (RI) are examined. The applicability of the model derived is justified by comparing with the experimental results from the literature. The latter are gathered from a system using poly(methyl methacrylate) as the host polymer, and methyl methacrylate and benzyl methacrylate as monomers. Our results show that the RI distribution of the fibers prepared by the co-extrusion process can be significantly adjusted by the essential parameters.