Aim of this study was to identify the molecular mechanisms of tanshinone IIA-induced apoptosis in chronic myelogenous leukemia (CML) cells. Cytotoxicity of tanshinone IIA was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Our data demonstrate that tanshinone IIA induced apoptosis by increasing the sub-G1 DNA contents and DNA fragmentation in KBM-5 CML cell line. In addition, tanshinone IIA significantly reduced mitochondrial membrane potential (MMP), mediated cytochrome c release from mitochondria and activated caspase-3 and 9, indicating mitochondria-dependent apoptosis by tanshinone IIA. Tanshinone IIA attenuated expression of several apoptosis-related proteins such as c-inhibitor of apoptosis protein (IAP) 2, Mcl-1(L) and Bcl-2. Interestingly, although tanshinone IIA notably enhanced the phosphorylation of both c-Jun N-terminal protein kinase (JNK) and p38, JNK inhibitor, but not p38 inhibitor, reversed tanshinone IIA-induced apoptosis. Our findings suggest that tanshinone IIA induces mitochondria-dependent apoptosis via activation of JNK in KBM 5 cells as a potent anti-cancer agent for CML therapy.