When two monomers with different densities and refractive indices are polymerized under a centrifugal force field, a cavity is generated in the rotational axis as a result of inherent volume shrinkage. Accordingly, an additional monomer-refilling process is necessary to compensate for the undesirable cavity. In this study, we modified the stepwise refilling process to an automatic process and have successfully fabricated a graded-index polymer optical fiber preform without a cavity. The process could also reduce the processing time and enhance the transmission speed of a polymer optical fiber compared with the stepwise process. (c) 2005 Wiley Periodicals, Inc.