A doped poly(methyl methacrylate) (PMMA)based graded-index plastic optical fiber (GI POF) with high mechanical strength is reported for the first time. Although the POF is generally believed to have a good mechanical flexibility even if it has a large-core diameter, such a high mechanical strength has been provided by making the polymer chains in the POF highly oriented in its axial direction. If such an orientation of polymer chains is eliminated, the POF becomes brittle, which is similar to silica-based fibers. On the other hand, too high an orientation of the polymer chains induces fiber deformation in a high-temperature atmosphere resulting from orientation relaxation. This study reports how high mechanical strengths such as the tensile strength and the large elongation are provided to the GI POF. By selecting the optimum heat-drawing conditions, the GI POF has a mechanical strength comparable to that of the commercially available step index (SI) POF. (C) 2003 Wiley Periodicals, Inc. J Appl Polyrn Sci 91: 404-409, 2004.