A series of novel phenoxazinone derivatives (1-6) were designed and synthesized for evaluating their antitumor activities. The antiproliferative activities of the prepared compounds against representative human neoplastic cell lines were evaluated by MTT assay. The results showed that most of them inhibited cell proliferation in a submicromolar to micromolar range.These compounds were also evaluated against KBv200 and MCF-7/Adr cell lines, which overexpress the MDR/P-glycoprotein drug efflux pump responsible for drug resistance, and had a more potential for resisting MDR than their lead compound APO.