Accumulating evidence suggests that cyclooxygenase (COX) family members are associated with a proliferative potential and apoptosis resistance of many cancers. COX-2 is an important molecular target for both therapy and prevention of malignancies. In vitro studies have revealed that treatment of various human epithelial cancer cell lines with specific COX-2 inhibitors induce apoptotic cell death. However, little is known about tumor cell lines of hematopoietic origin. The effects of COX-2 specitic inhibitor-celecoxib on proliferation and apoptosis on K562 cells are sought to be determined. K562 cells were exposed to increasing concentration of celecoxib (0, 10, 20, 40, 80, 160 μmol/L). The growth inhibition was evaluated by trypan blue dye exclusion, MTT assay and colony- formation inhibiting test. Apoptosis was determined by DNA ladder in agarose gel. The morphology apoptotic cells was identified by AO/EB staining combined with fluorescence microscopy observation, and by caspase-3 assay. The percentage of apoptotic cells was determined by flow cytometry. A blocking experiment for caspase-3 was carried out by incubating K562 cells together with DEVD-fmk. In addition, whether there is an expression of COX-2 mRNA or protein in K562 cells were investigated by RT-PCR and Western blot. Celecoxib(40～160 μmol/L) can effectively inhibit the proliferation and induce apoptosis of K562 cells in a dose-dependent manner. IC₅₀ of celecoxib for inhibiting cells proliferation was 46 μmol/L. The regulation of apoptosis induction was related to an upregulated caspase-3 expression and activation. When caspase-3 activity was blocked by DEVD-fmk, the effect of celecoxib on apoptosis induction was partly abolished. Importantly, It is first demonstrated that K562 cells indeed exists in COX-2 expression both mRNA and protein (IL-1β inducement assay and RT-PCR/Western blot confirmation), celecoxib (80～160 μmol/L) could significantly down-regulate the expression of COX-2 mRNA and protein. In conclusion, these data indicated that a selective COX-2 inhibitor celelcoxib inhibits cell proliferation, induces apoptosis in human K562 cells, thus suggesting that COX-2 inhibitors may be a promising strategy in treatment of CML.