Stiffness reduction of poly (methyl methacrylate) (PMMA) subjected to alternating bending deformation is measured as a function of the number of loading cycle, and effects of temperature on the stiffness reduction process are investigated in the present study. Reduced method is found to be applicable between temperature and the number of loading cycle. Apparent activation energy estimated from reduced method is 23±5kcal/mol, which is comparable to that of viscoelastic β-relaxation process for PMMA. The activation energy estimated from the temperature dependence of craze density, 9±4kcal/mol is lower than that estimated on the basis of the reduced method, and this fact suggests the stiffness reduction may be attributed to the characteristics of PMMA rather than craze formation. Effects of subjecting the specimens to cyclic bending on viscoelastic properties and tensile strength are also examined.