Abstract: Objective To study apoptosis effect of matrine (Mat) on acute promyelocytic leukemia cells (NB4) and its mechanism.Methods NB4 cells were treated with Mat of different concentrations (0.25,0.5,and 1.0 mg·mL^-1) for 24,48,and 72 hours.The morphologic changes of the cells were observed with Hoechst 33258 staining and proliferation inhibition of NB4 cells was determined with 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) assay.Cell cycle and apoptosis rate were analyzed by flow cytometry with annexin V-fluorescein isothiocyanate/propidium iodide (FITC/PI) double staining.The relative activity of caspase-3 and caspase-8 were examined with spectrophotometry assay.Results Mat could significantly inhibit the proliferation and induce apoptosis of NB4 cells in a dose- and time-dependent manner compared with the normal control (P < 0.05).Morphology change observation revealed characteristics of cell apoptosis.Mat at the doses of 0.5 and 1.0 mg·mL^-1 could block NB4 cell cycle at G₀/G₁ stage (57.91% and 83.00%) and reduce the proportion of the cells at S stage (40.95% and 16.10%),and significantly increase the activity of caspase-3(77.41±4.01 and 111.78±4.05).Further analysis indicated that Mat at the does of 1.0 mg·mL^-1 could increase the activity of caspase-8(44.98±7.63)(P < 0.05).Conclusion Mat could inhibit the proliferation and induce apoptosis of NB4 cells; the mechanism of the effects may be related to the arrest of G₀/G₁ phase,inhibition of DNA synthesis,and activation of caspase-3 and caspase-8 pathway.