Acrylic resin denture base fracture is a primary clinical mode failure after denture delivery due to high impact forces or repetitive stress rate. The fracture can result from flexural fatigue or degradation of the base material. Therefore, it is necessary to increase the strength of denture base resins for its longevity. The purpose of this study was to evaluate the effect of 2.5 and 10 weight % silane-treated alumina oxide reinforcement on the flexural strength of heat-polymerized acrylic resins. Thirty rectangular (65x10x3 mm.³) (ISO standard 1567) heat-polymerized acrylic resins were fabricated (n: 10). The control group was an intact heat-polymerized acrylic resin group. The two experimental groups were reinforced with 2.5 and 10 weight % silane-treated aluminum oxide particles (50-70 µm.). The specimens were tested for flexural strength using a three-point bending test. One-way ANOVA and Tukey Honestly Significant Difference (HSD) were used for statistical analysis. Flexural strength of 10 weight % of silane-treated aluminum oxide reinforced groups (110.45 MPa) significantly increased compared to control (99.25 MPa) (p-value<0.005), respectively, even though flexural strength of 2.5 weight % of silane-treated aluminum oxide reinforced groups (99.27 MPa) did not increase significantly compared with the control (99.25 MPa) (p-value<0.005). In conclusion, changing the percentage by weight of silane-treated alumina reinforced affected the flexural strength of heat-polymerized acrylic resins, as the flexural strength increased significantly when 10 weight % silane-treated alumina fillers were added.