Objective The therapeutic effect of chronic myeloid leukemia (CML) is undesirable. In order to find a new sensitive drug for CML, this study aims at exploring the effects of tetra arsenic tetra sulfide (As 4S 4) on human leukemia K562 cells. Methods The viability of K562 cells, represented by absorbance, was measured by MTS assay. The cells morphological changes were determined by Wright's staining and Hoechst33342 assay. The cell apoptosis was evaluated by DNA agarose gel electrophoresis and the cell apoptosis rate was measured by flow cytometry. Results The cell viability decreased significantly being cultured with 5.0 μmol/L As 4S 4, STI571 and herbimycin for 72 hrs (with absorbances of 0.32 ± 0.04 , 0.49 ± 0.01 and 0.69 ± 0.02 , respectively). As 4S 4 and STI571 had more inhibition on the K562 cells viability than herbimycin (P< 0.01 ), and there was no statistically significant difference between that of As 4S 4 and STI571. The effects of 1.0, 5.0 and 10.0 μmol/L As 4S 4 on K562 cells were time dependent. When the concentration was lower than 1.0 μmol/L, As 4S 4 had little effect on K562 cells. After being cultured with 2.0 μmol/L As 4S 4 for 24 to 48 hrs, typical morphological changes of apoptosis appeared in the K562 cells. After being cultured with 5.0 μmol/L As 4S 4, STI571 and herbimycin for 72 hrs, the apoptosis rate of K562 cells were 68.8%, 56.7% and 35.5%, respectively. When the concentrations of As 4S 4 changed from 2.0 to 3.0 μmol/L, the apoptosis rate increased from 25.7% to 45.3%. There was no significant difference between the apoptosis rate of K562 cells induced by 5.0 and 10.0 μmol/L As 4S 4. Conclusions As 4S 4 with the concentration of 2.0 μmol/L could inhibit the growth of K562 cells efficiently through inducing apoptosis.