Purpose:To evaluate the effect of adding Al 2 O 3 , SiO 2 , and TiO 2 nanoparticles in ratios of 1, 3, and 5 wt% to different acrylic resins on flexural strength. Materials and Methods: A total of 210 specimens were prepared in 30 groups (n = 7/group) (Control, 1% Al 2 O 3 , 3% Al 2 O 3 , 5% Al 2 O 3 , 1% SiO 2 , 3% SiO 2 , 5% SiO 2 , 1% TiO 2 , 3% TiO 2 , 5% TiO 2 ). The specimens were polished with 200-, 400-, and 600-grit abrasive paper to provide a standard surface before testing and then suspended in distilled water for 30 days. Flexural strength was measured via three-point bending tests. Subsequently, SEM analysis was performed for one specimen from each group. Homogeneity of data was assessed by Kolmogov-Smirnov test followed by two-way ANOVA and Tukey HSD tests (α = 0.05). Results: There was a significant increase in the flexural strength of polymethylmethacrylate (PMMA) after addition of 1% nanoparticles in both heat-polymerized and autopolymerized acrylic resins (p ˂ 0.05). The flexural strength values of the groups to which Al 2 O 3 and TiO 2 nanoparticles were added exceeded those of the group with SiO 2 addition (p ˂ 0.05). The electron microscopy images revealed that the nanoparticles were more homogeneously dispersed in PMMA with higher flexural strength. Conclusions:The mechanical properties of PMMA can be improved by the addition of nanoparticles to PMMA; however, the flexural strength values of PMMA decrease with the addition of nanoparticles at higher percentages (3-5%). Hence, the ideal filler ratio corresponds to 1%.

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