© 2018, Nihon University, School of Dentistry. All rights reserved.The aim of the present study is to evaluate the transverse strength, modulus of elasticity, surface roughness, hardness, and water sorption/solubility of nanoparticle zirconium oxide (nano-ZrO2) added to heat-cured poly(methyl methacrylate) (PMMA) after thermocycling. The specimens were divided into four groups according to the ratio of nano-ZrO2 addition to heat-cured PMMA; group 1: 5% nano-ZrO2; group 2: 10% nano-ZrO2; group 3: 20% nano-ZrO2, and group 4 (control): PMMA without nano-ZrO2. A statistically significant decrease in transverse strength was observed by all additional rates of nano-ZrO2 compared with group 4 (control; P < 0.025). When hardness results were evaluated, groups 1 and 2 showed statistically higher values compared with groups 3 and 4 (P < 0.025). The surface roughness of group 3 was statistically higher than the other groups (P < 0.025), but it was within the clinically acceptable limits. As the nano-ZrO2 addition rate increased, water sorption/solubility values of the specimens also increased. Although nano-ZrO2 addition had an adverse effect on some mechanical and physical properties of heat-cured PMMA, it increased hardness values, implying that the addition of nano-ZrO2 would contribute positively to some mechanical properties of PMMA denture base material when nano-ZrO2 was homogeneously distributed in PMMA.