Objective:To investigate the reversing effects and mechanism of ursolic acid on drug resistance in nude mice bearing ovarian cancer stem cells. MethodsThe human ovarian cancer stem cells were enriched and collected from serum free cells suspension of Skov3 cells. Then the cells were inoculated subcutaneously into nude mice. The nude mice were treated with ursolic acid (high, medium and low dose), cisplatin, andcisplatin plus ursolic acid respectively for 14 continuous days. Tumor growth curve and inhibitory rate were calculated by daily measurement of tumor volume. Cell morphology and structure were observed by HE staining; ABCG2 expression was detected by immunohistochemistry and cell apoptosis was detected by the TUNEL method.  Results:Ursolic acid inhibited tumor growth in a dose-dependent manner, and ursolic acid plus cisplatin group was superior to cisplatin group. Ursolic acid inhibited the expression of ABCG2, a drug rsisitance index, and promoted apoptosis of cancer stem cells, improved the ability of cisplatin-induced apoptosis.  Conclusion:Ursolic acid can inhibit the tumor growth in nude mice transplanted with ovarian cancer stem cells by decreasing ABCG2 expression, promoting cell apoptosis, enhancing cisplatin-induced apoptosis.