Objective To study the effects of COX-2 selective inhibitor celecoxib on the apoptosis of acute promyelocytic leuke-mia NB4 cell line,and to investigate its apoptosis mechanisms.Methods The expression of COX-2 mRNA in different cell lines was detected by reverse transcript polymerase chain reaction(RT-PCR).After treatment of NB4 with different doses of celecoxib,the in-hibition of NB4 growth was assayed by MTT,and the DNA fragmentation was examined by the DNA ladder test.The level of Bcl-2 protein expression was assayed by the flow cytometry.Results As compared with the no-medication treatment group,the DNA ladder fragments became more and more obvious after the treatment by different doses of celecoxib.The expression rates of Bcl-2 protein in the different doses of celecoxib groups (25,50,100 μmol/L)were (71.69 ±1.65 )%,(34.51 ±2.53)% and (29.28 ± 2.38)% respectively,compared with the Bcl-2 protein expression rate (85.34±2.89%)in the blank control group,the expression rate of Bcl-2 protein in different doses of celecoxib groups(50,100 μmol/L )was significantly decreased(P <0.05 ).Conclusion Celecoxib as COX-2 selective inhibitor could evidently induce the apoptosis of NB4 cells by down-regulating the expression of Bcl-2 protein in NB4 cells.