The graded-index polymer optical fiber (GI-POF) is a promising candidate for high-speed communication medium for very short reach networks such as home networks and office local area networks (LANs). It is well-known that there is an optimum refractive index profile to maximize bandwidth properties of GI-POF when the profile is expressed by the power law equation. The interfacial-gel polymerization technique, one of the major conventional fabrication methods of GI-POF, has many processes, and the formation mechanism of refractive index profiles intricately depends on many parameters of fabrication conditions. Thus is very difficult to control and form an optimum radial refractive index profile in the GI-POF. Recently, the coextrusion process has been proposed for mass production of GI-POF. In this process, the forming process of refractive index profiles can be expressed by a simple advection-diffusion equation. Therefore, we investigated the general methodology to control refractive index profiles by numerically solving the advection-diffusion equation. On the basis of our data, it was determined that refractive index profiles could be controlled by adjusting diffusion temperature and core ratio.