In this study the leaves of the Thai noni/Yor,(Morinda citrifolia Linn.) were extracted by several methods and evaluated against human cancer cell lines: KB (human epidermoid carcinoma), HeLa (human cervical carcinoma), MCF-7 (human breast carcinoma) and HepG2 (human hepatocellular carcinoma) cell lines as well as a Vero (African green monkey kidney) cell line, employing the MTT colorimetric method, comparing it to damnacanthal, rutin, and scopoletin. The dichloromethane extract of the fresh leaf showed a better inhibitory effect against KB and HeLa cells with IC50 values of 21.67 and 68.50 µg/ml, respectively. The dichloromethane extract of dried leaves revealed cytotoxicity against the KB cell line with an IC50 value of 39.00 µg/ml. Other extracts, as well as rutin and scopoletin, showed reduced anti-proliferative effects on all cancer cell lines (IC50 103 to over 600µg/ml). Interestingly, the damnacanthal had potent cytotoxicity against all cancer cell lines and Vero cell lines. These results suggest Thai noni extracts may be safer than the pure compounds, due to their higher safety ratios, which is a good indicator for possible cancer treatment. Several non-aqueous extracts from the leaves showed antioxidant properties, giving IC50 values of 0.20-0.35 mg/ml. It can be concluded the leaves of M. citrifolia may have benefit as a food supplement for chemoprevention against epidermoid and cervical cancers.