Abstract : auto-polymerized acrylic resins are widely substances utilized for the fabrication of orthodontic appliances. Such substances have poor mechanical properties and prone to fracture. The purpose of the current study was to evaluate the effect of incorporating titanium dioxide nanoparticles to auto- polymerized acrylic resins in terms of the surface hardness. thirty specimens were constructed in total. They were fabricated from auto-polymerized acrylic resins. Ten specimens were considered as control and the other 20 specimens were reinforced with the titanium dioxide powder at 1% and 2% respectively. All specimens were subjected to the hardness tester. The SPSS version 20 was used to analyze the data, and the Tukey test was used for the comparison among the 3 groups. The results indicated that the titanium dioxide improved the surface hardness strength of acrylic resins. No significant differences were found between control and TiO2 1 groups; TiO21 and TiO2 2 groups where (P>0.05). on the other hand, there was significant difference between control and TiO2 2 groups where P≤0.001.the adding of titanium dioxide increases the surface hardness of auto-polymerized acrylic resins