Abstract: Objective :To study the mechanism of quercetin induce apoptosis in K562 cells after the leukaemia cell line I562 were treated by quercetin, and probe into the enzyme activity changes of Caspase-3 in the process. Methods :K562 was cultured in vitro and then studied by MTT to measure ICSO_ so as to observe the cytotoxicity of quercetin; The morphology changes of the nucleons were observed by Hoechst33342/PI staining, and the enzyme activity changes of Caspase-3 in the process of quercetin induced I562 apoptosis were detected by CaspACET assay system. Results :when the concentrations of quercetin were 2x 10-5 mol/L、4x 10-5 mol/L、8x10-5 mol/L, the growth of cell was inhibited remarkably in the way of dosage dependence. After the I562 cells were treated by different concen trations of quercetin for 48 hours, the ICSO was 4x10 mol/L by MTT experiments, distinct morphology changes of cell apoptosis such as karyopyknosis and conglomeration were observed by Hoechst33342/PI staining. The enzyme activity of Caspase-3 rise remarkably (P<0.05), and the raise degree positively correlated with drug administration dosage. Conclusions :quercetin exerts its anti-cancer effect by inducing the leukaemia cell apoptosis, and the Caspase-3 pathway is one of the mechanisms.