A novel coextrusion method has been introduced for the fabrication of gradient-index plastic optical fibers (GI-POFs). In this method, the radial diffusion of a refractive-index-modifying dopant in a polymer matrix is enhanced significantly by a new die design concept. A theoretical analysis indicates that this new method can substantially reduce the material residence time required to obtain a smooth refractive-index profile, thereby increasing the output rate to a much higher level than possible with the conventional methods. An experiment using a poly(methyl methacrylate) (PMMA) with diphenyl sulfide (DPS) and diphenyl sulfoxide (DPSO) as the refractive-index-modifying dopants has shown an excellent agreement with the theoretical prediction, suggesting that the new coextrusion method is a viable way to manufacture a GI-POF with the bandwidth higher than 700 Mbits/s at a distance of 100 m.