Nano composite materials based on polymers are widely used in restoration martials like dental. Poly methyl methacrylate (PMMA) is one of the most used polymers as dental material. PMMA has disadvantages such as low flexural strength properties and impact strength. In this paper, the influences of additive aluminum oxide and hydroxyapatite nanoparticles on the mechanical and strength properties of PMMA (including flexural strength, impact strength, surface hardness and shrinkage behavior) is studied experimentally. For this purpose, nine standard mechanical testing samples of pure PMMA, PMMA/5HA and PMMA/10HA with various amounts of Nano aluminum oxide (3,6,8 wt.%) were prepared. The results showed that the mechanical properties of hybrid Nano-composites were significantly improved in comparison with the pure samples in a way that optimal Nano-composite with convenience flexural properties and impact strength is obtained. Moreover, the use of nanoparticles results in shrinkage reduction of hybrid Nano-composites in comparison with pure PMMA.