The present study was undertaken to investigate the phytochemical composition and antioxidant potential of methanolic extract (leaves) of Thuja occidentalis. The total phenolic and flavonoid content was determined using Folin-ciocalteau and Aluminum chloride method. The antioxidant activity was evaluated using DPPH, ABTS, NO and lipid peroxidation (LPO) assays. The results revealed that methanolic extract of T.occidentalis had an appreciable antioxidant activity. The IC50 value for DPPH, ABTS, NO and LPO are 0.47, 0.42, 0.82 and 0.42 mg/ml, respectively. The TPC and TFC was calculated to be 135.32 mg/g GAE and 3.46 mg/g QE. The antioxidant activity demonstrated by T. occidentalis may be due to its high phenolic content.