Heating at high temperatures creates a large change in the chemical properties of oils. Due to effects of oils on health, further research is necessary to choose the best type and quality of oils. Extra virgin olive oil is considered one of the best due to the beneficial effects of dietary oils. The aim of this study was to compare the thermal stability of Iranian and foreign extra virgin olive oil. Eight samples of olive oil were tested in this study. Oils were heated at 120oC for 4 h to evaluate the thermal stability, were sampled every 2 hours. Fatty acid composition, Acid value, Peroxide value, Anisidine value, Totox value, Rancimat oxidative stability test, was conducted in accordance with Iranian national standards. Results showed that oleic acid, the major fatty acid in olive oil, was between 69 to 74 percent. There was a significant relationship between time and acid value (P=0.013), peroxide value (P?0.001), anisidine value (P?0.001), totox value (P?0.001). There was not observed any significant relationship between changes in oil and acid value, peroxide value, anisidine value, but with totox value (P=0.003) a significant relationship was observed. Interpretation of the data suggests that the thermal process changes the index of acidity, peroxide, anisidine and totox. The changes will further increase with time. According to the obtained results, foreign extra virgin olive oils are better and are more resistant to heat than Iranian ones.