Aim: The aim of this study was to explore the effects and mechanism of berbamine on imatinib-resistant BCR-ABL-positive human leukemia K562 (K562-r) cells in vitro and in vivo. Methods: Cell viability was measured by MTT assay, and apoptotic morphology changes were detected by fluorescence microscopy. The apoptosis rate was measured by flow cytometric assay, mdr-1 mRNA levels were determined by RT-PCR. Bcl-2 family proteins, cytochrome c( cyt C), poly (ADP-ribose) polymerase (PARP), and P-glycoprotein were detected by Weste blot. BALB/c nu/nu mice were injected with K562-r ceils subcutaneously. Tumor-bearing mice were treated intravenously with berbamine.Results: MTT tests revealed that berbamine significantly inhibited K562-r cell proliferation and increased the chemo-sensitivity of Kf62-r cells to imatirtib. The apoptosis rate was significantly increased following treatment with 21.2 μmol/L berbamine; formation of typical apoptotic blebs was apparent, as observed by fluorescence microscopy. Expression levels of mdr-1 mRNA and P-gp protein were high in untreated K562-r cells and significantly down-regulated by berbamine treatment. Berbamine-treated K562-r cells also exhibited down-regulated expression of the anti-apoptotic proteins Bcl-2 and Bcl-xL, up-regulated expression of the apoptotic proteins Box and cytoplasmic cyt C, and stimulated proteolytic cleavage of PARP. In addition, berbamine also suppressed the growth of K562-r xenotransplanted tumors in vivo. Conclusion: Berbamine inhibited proliferation of K562-r cells both in vitro and in vivo. Berbamine-induced apoptosis in K562-r cells appeared to occur through a mechanism involving Bcl-2 family proteins, as well as mdr-1 mRNA and P-gp pro- tein. 13erbamine in combination with imatirtib restored the chemo-sensitivity of K562-r cells to imatinib. Our findings sug-gest that berbamine may be useful in treating imatinib-resistant CML patients.