A closed extrusion process combined with a core-shell separation die design is developed in this study for preparing gradient index (GRIN) polymer fibers with a quadratic distribution of the refractive index. The material system used in this investigation is methyl methacrylate (MMA, n = 1.49) and benzyl methacrylate (BzMA, n = 1.568). The refractive index differences between the center and the periphery (an) of the prepared polymer fibers increase from 0.0115 to 0.020 when BzMA in the reactant mixtures increases from 17 to 28%. This finding would suggest that increasing the high refractive index monomer in the reactant mixture increased Delta n. The an values decrease from 0.018 to 0.0135 when the diffusion zone temperature increases from 70 to 90 degrees C. The diffusion rates of the monomers increase with the temperature, thereby causing BzMA and MMA to distribute more uniformly at a higher temperature than at a lower one. This uniform distribution leads to the decrease of an with an increasing temperature. The prepared GRIN polymer fibers have potential applications as imaging lenses for scanners, fax machines, and copiers.