A poly(methyl methacrylate) (PMMA) base gradient-index plastic optical fiber (GI-POF) was fabricated by the diffusion-assisted coextrusion process using diphenyl sulfide (DPS) as a refractive index modifying dopant. In the diffusion-assisted coextrusion process, PMMA containing DPS and pure PMMA were coextruded concentrically as the core and cladding, respectively. Subsequently, DPS contained in the core was allowed to diffuse in the diffusion zone of a coextrusion die forming a gradually varying concentration gradient in the radial direction. Thereby, a refractive index profile was created. The refractive index profile was then characterized by measuring the concentration profile of DPS using FT-IR spectroscopy. The measured profiles showed a good agreement with the predictions of the theoretical model reported previously (Ind. Eng. Chem. Res. 2001,40, 3740). Because the concentration profile of the dopant is strongly affected by its diffusivity, the refractive index profile could be controlled by adjusting the processing temperature and/or the residence time of the materials in the coextrusion die. The results of this study suggest that the diffusion-assisted coextrusion process is a viable method to fabricate a GI-POF with a bandwidth higher than 600 Mbits/s at a distance of 100 in.