Background: In recent years, natural products, especially natural antioxidants, play an important role in various fields. In fact, Lippia citriodora is one of the medicinal plants which is cultivated in Iran and it may be has potential antioxidant activity. Objective: The chemical composition of Lippia citriodora essential oil (LCEO) has been determined. The total phenolic content and antioxidative properties of LCEO were determined. Finally, the effect of the thermal treatment (80, 120, 180ºC) for 1, 2 and 3 hours on the antioxidant activity (AOA) and the chemical composition of LCEO were studied. Methods: The unheated and heated LCEO components were identified by GC/MS. The antioxidant activity (AOA) of LCEO was determined by two complementary test methods, namely DPPH free radical-scavenging and β-carotene bleaching. Results: LCEO had different chemical components such as R-curcumene (14.1 %), caryophyllene oxide (6.6 %), 6-methyl-5-hepten-2-one (7.1 %), and spathulenol (7.0 %). The IC50 of LCEO was compared with that of synthetic antioxidant (BHT). Antioxidant activities of LCEO at different concentrations (0.6- 2.5 mg/ml) were determined by β-carotene bleaching method. The antioxidant activities of LCEO at this range of concentrations were 1.5 - 68.0 %. After heating up to 180ºC, the essential oil showed a significantly higher free radical-scavenger activity and evident changes in its chemical composition. Conclusion: The study can be further extended to exploit not only the phenolic extracts but also the residual phenolic constituents associated with this herbal medicine as health supplement.