Abstract: Objective To investigate regulating effects of Herba Saginae Japonicae(HSJ) on apoptosis and cell cycle of K562 cell strain in vitro.Methods The inhibited proliferation of K562 cells was measured with methyl thiazolyl tetrazolium(MTT).The effect of HSJ on cell growth was analzyed with trypan blue dye exclusion method.The HSJ-induced apoptosis of K562 cells was detected with annexinV-FITC/PI double staining method.The cell cycle phase of K562 cells was determined with DNA ploid analysis.Results There was a positive relation between inhibitive effect of HSJ on K562 cell proliferation and the concentration of HSJ.The 50% inhibitory concentration(IC₅₀) of HSJ was 1.35 mg/ml with the treatment of HSJ for 48 hours.The apoptotic ratios of K562 cells were 8.39±0.81%,8.88±1.12%,and 13.42±2.04% treatd with HSJ of 1.0,2.0,and 4.0 mg/ml,respectively,with a significant difference compared with that of the control group(4.60±1.09 %)(P<0.05).Compared with control,HSJ increased the percentage of the cells in G2 phase(P<0.05),decreased the percentage of cells in S phase and the proliferous index(PI)(P<0.05,P<0.05).Cell cycle of K562 cells with HSJ treatment were blocked in G2 phase.Conclusion HSJ can inhibit the proliferation of K562 cells and the mechanism maybe involve the induced cell apoptosis and blocked G2 phase of K562 cells.