Background: Chamuangone has been isolated from Garcinia cowa leaves and exhibited various biological activities, i.e., antibacterial, anti-Leishmania major, and cytotoxic activity against cancer cells. n-Hexane has been reported to be the most suitable solvent for extraction of chamuangone. Objectives: Some vegetable oils were determined as an alternative green solvent for extraction of an anticancer compound, chamuangone from G. cowa leaf. The chamuangone-enriched extract was standardized and evaluated for cytotoxic activity against human cancer cell lines. Materials and Methods: Microwave-assisted extraction and high-performance liquid chromatography were used for extraction and standardization. The cytotoxic activity was determined using a sulforhodamine B assay. Results: The chamuangone-enriched extract was obtained using rice bran oil as the alternative green solvent and standardized to contain 1.97 mg/mL chamuangone. The extract exhibited cytotoxic activity against human lung adenocarcinoma, human breast adenocarcinoma, and human colorectal adenocarcinoma cell lines, with IC50 values of 15.3, 15.9, and 12.8 μg/mL, respectively, but was nontoxic to human gingival fibroblasts, a normal cell line, at a concentration of 50 μg/mL. Moreover, the extract contained several natural antioxidants, including α-tocopherol (76.7 mg/100 g), γ-oryzanol (cycloartenol ferulate: 67.1 μg/mL and 24-methylenecycloartanol ferulate: 85.6 μg/mL), and antioxidant capacity determined as ascorbic acid (258.7-mM ascorbic acid equivalent per gram). Conclusion: Based on these findings, the chamuangone-enriched extract may be considered as a novel functional food in cancer chemopreventive action.