The properties of poly(methyl methacrylate) (PMMA)-based graded-index polymer optical fiber (GI POF), including the thermal stability, thermal humidity, and mechanical properties, were studied for polymer optical fiber research and applications. The glass-transition temperature of the fiber core was 103degreesC in the presence of the dopant, which was close to that of the PMMA matrix without the dopant. A special refractive-index profile derived from the distribution of the dopant was stable at 60degreesC. Moreover, GI POF exhibited good mechanical properties. The excellent performance indicated that GI POF could be applied not only for indoor use but also for outdoor use. However, PMMA-based GI POF exhibited poor hot-water/ humidity resistance. (C) 2003 Wiley Periodicals, Inc.