Background: Thymus daenensis Celak. is one of the medicinal plants which cultivated in Iran. This plant involve high amount of polyphenols including thymol and carvacrol, which were reported to possess the high antioxidant activity. Objective: Antioxidative activity (AOA) and chemical constituents of Thymus daenensis essential oil (TDEO) were examined. Also, the effect of thermal treatments (80,120 and 180 oC) for 1, 2 and 3 hours on the AOA and chemical constituents of TDEO was studied. Methods: The chemical constituents of Thymus daenensis essential oil was analyzed by GC/MS. Antioxidant activity (AOA) of TDEO was determined by two complementary test methods, namely DPPH free radical-scavenging and β-carotene bleaching. Results: TDEO had different chemical components, including thymol (54.7%), carvacrol (5.0%), linalool (1.9%), 1-octene-3-ol (1.6%), p-cymene (11.3%), terpin-4-ol (1.5%), γ-terpinene (12.9%), α-terpinene (2.0%), myrcene (1.1%), β-caryophelene (2.6%) and borneol (1.1%). The IC50 of TDEO was compared with synthetic antioxidant (BHT). Antioxidant activities of TDEO at different concentrations (0.1- 3.1 mg/ml) were determined by β-carotene bleaching method. Antioxidant activity of TDEO at these range of concentration were 20 - 96.0 %. After heating up to 180oC, essential oil showed a significantly higher free radical-scavenger activity and evident changes in its chemical composition. Conclusion: Owing to this property, the studies 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 and nutraceutical.