BACKGROUND Silicone elastomer is the most used material for fabricating maxillofacial prostheses, but the material has low tensile and tear strength and insufficient elasticity. Whether the addition of zinc oxide nanoparticles will improve these properties is unclear. OBJECTIVE The purpose of this in vitro study was to evaluate the effect of adding different concentrations of zinc oxide (ZnO) nanoparticles on the clinically critical mechanical properties of a maxillofacial silicone. METHODS Nano-ZnO was added in concentrations of 1%, 2%, 3%, and 5% by weight to Cosmesil M511 High Temperature Vulcanization (HTV) silicone elastomer. Silicone without nano-ZnO or ethanol served as a conventional group, while silicone without nano-ZnO and with ethanol served as the control group. Tensile strength and elongation tests were done according to International Organization for Standardization (ISO) 37. A tear strength test was done according to ISO 34-1. A Shore A hardness test was done according to ISO 7619. In total 144 specimens were fabricated, with 48 specimens for each test. Field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) tests were used to assess the efficiency of the dispersion method, monitoring the particle size of nano-ZnO. Statistical analysis used 1-way ANOVA and the Tukey post hoc test (α=.05). RESULTS FESEM showed a homogenous dispersion of ZnO nanoparticles within the silicone matrix. XRD and FESEM showed a reduction in cluster size of nano-ZnO after sonication. The FTIR-ATR test revealed no significant difference between the conventional and the control group (P>.05). The 3%ZnO demonstrated significantly higher tensile strength, elongation percentage, and tear strength than the control group (P<.05). Shore A hardness showed a significant decrease between the conventional and control group. Hardness increased directly and proportionally to an increase in the concentration of nanofiller. CONCLUSIONS Sonication of nano-ZnO in ethanol represented an effective and straightforward way to disperse nano-ZnO in a silicone elastomer matrix. This improved the quality of the nanocomposite without affecting the base material and without the need for a coupling agent or addition of a third material. The overall mechanical properties of the M511 maxillofacial silicone elastomer improved most with a 3%ZnO concentration.