In this study the effect of thermocycling on the mechanical properties of a commercial dental nano-composite (Filtek Z350 XT, 3M ESPE, Germany) was investigated by using the nano-indentation experiment. For this purpose some disk specimens, each of the diameter 10 mm and the thickness 4 mm were prepared. Half of the specimens were stored at room temperature and the other half were thermocycled in distilled water for 1000 cycles between temperatures 5°C and 55°C. After the sample preparation, the nano-indentation test was applied on both non-themocycled and thermocycled specimens by Triboscope setup. Then the modulus of elasticity and hardness of the test samples were calculated from the Oliver-Pharr’s method using the data obtained from the nano-indentation experiments. Using the independent-samples t-test, the mechanical properties obtained from the non-thermocycled and thermocycled samples were compared. The results indicate that thermocycling process increases the modulus of elasticity and hardness of the dental nano-composite.