In vivo experiment has been conducted to observe the preventive role of Thuja occidentalis Linn (leaves) against 7, 12 dimethylbenz(a) anthracene (DMBA) -induced mammary cancer. Ethyl acetate (EtOAc) and methanolic (MeOH) extracts in two doses (5 and 10 mg/kg body weight) of the plant were tested for DMBA-induced Indian Cancer Research Centre (ICRC) mice mammary carcinoma in terms of tumor weight, volume, life span, histological variation and oxidative stress against the reference drug doxorubicin using standard animal protocol. EtOAc extract (10 mg/kg body weight) of the plant exhibits reduction of tumor weight (39%), tumor volume (50%), reduced glutathione (GSH) (83%) and malignant cells compared to cancerous control group while the increase in body weight and life span in comparison with cancerous control and doxorubicin-treated group. EtOAc extract being most potent extract has been subjected to detailed chromatographic separation. The most potent chromatographic fraction exhibits the presence of flavonoidal unit. Structural elucidation of bioactive principle is in progress. It is inferred that the plant T. occidentalis (leaves) possess significant potential for phytopreventive bioefficacy against DMBA-induced mammary carcinogenesis.