This study was purposed to explore the anti-leukemic mechanism of quercetin (Que) in vivo and it enhancing chemotherapeutic effect of adriamycin (ADR) by establishing the quercetin-treated P388 transplanted nude mouse model. The P388 leukemic cells in logarithmic growth phase were taken and injected subcutaneously into BALB/c nude mice so as to establish the leukemia-transplanted nude mouse model. The model mice were treated by quercetin, ADR and their combination, and the survival changes of model mice were observed, the hemogram and peripheral blood cell count examination were performed regularly; the cell cycle was detected and the influence of quercetin on cell proliferation was analyzed by flow cytometry; the caspase-3 protein expression level was detected by ELISA; the mRNA and protein changes of NF-κB, BCL-2, BAX were measured by real-time quantitative flourascence PCR and Western blot respectively. The results indicated that the quercetin and adriamycin could significantly prolong the survival of P388 leukemia nude mice, and their combination displayed significantly prolonged effect. Quercetin and adriamycin alone or in their combination could reduce the ratio of G0/G1 phase in mice, the cell ratio in S phase and G2/M phase increased, and the effects of the combination group were more significant than that of the single agent groups. Quercetin could activate caspase-3 and promote leukemic cell apoptosis. Meanwhile, quercetin could down-regulate the expression of BCL-2 and NF-κB gene, and up-regulate the expression of BAX gene. It is concluded that through modulating the expression of apoptosis-related genes, the quercetin can inhibit leukemia cell proliferation, promote apoptosis, and enhance the chemotherapeutic effects of adriamycin. These results provide some valuable data for further research and development of quercetin as a new and effective anti-leukemic drug.