It was shown how high mechanical strength should be provided in the high numerical aperture (NA) graded-index plastic optical fiber (GI POF). In this newly developed GI POF, a copolymer of methyl methacrylate (MMA) and 2,2,2-trifluoroethyl methacrylate (3FMA) was used to increase the NA. The GI POF we proposed previously was composed of a PMMA homopolymer cladding and a doped PMMA core. It was previously shown that substituting the copolymer P(MMA-3FMA) for the PMMA as the cladding material made little change in the fiber's light-transmission characteristics. This study focused on mechanical flexibility, which is one of the most important advantages of the POR It was found that the P(MMA-3FMA)clad GI POF had almost the same or superior mechanical strength in addition to the excellent light-propagation characteristics. It was also found that such excellent mechanical properties were achieved using a small dopant concentration and optimum heat-drawing conditions. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 410-416, 2004.