Objectives: The purposes of this study were to evaluate the strength and dimensional stability of PMMA reinforced with continuous silk fibers. Methods: The impact and transverse strengths of heat polymerized PMMA reinforced with 1 to 4 % silk fibers by weight were evaluated and compared with those of non-reinforced heat polymerized PMMA (10 specimens/group). The fibers were placed longitudinally in the middle of PMMA specimens. All specimens were immersed in distilled water at 37oC for 14 days prior to the tests. The fracture characteristics of the reinforced specimens were also investigated under a scanning electron microscope. Subsequently, the most effective reinforcement fiber concentration was used to prepare specimens for dimensional stability evaluation up to 14 days and compared with non-reinforced PMMA (10 specimens/group). Results: (1) The impact strength of reinforced PMMA increased approximately 8 times when compared with that of the control group (from 0.44 to 3.46 ft-lb, p = 0.000). There was no significant difference in impact strength among the reinforced groups. (2) The transverse strength of 1%- and 2%-fiber groups were slightly higher than that of control group (approximately 90MPa vs 80MPa, p=0.002). On the contrary, the transverse strength of 3%- and 4%-fiber groups were significantly lower than that of control group (approximately 30MPa vs 80MPa, p=0.000). (3) SEM analysis showed a “pullout” phenomenon of fibers from the fractured PMMA pieces. (4) The dimensional stability up to 14 days of 1%-fiber and control groups were not significantly different (0.14% vs 0.13% expansion, respectively, p=0.062). Conclusions: The optimum continuous silk fiber concentrations used in PMMA reinforcement were 1% and 2% by weight. The 1% silk fiber reinforcement did not significantly alter the dimensional stability of PMMA. (This study was supported by Graduate School, Prince of Songkla University).