AIM To study the effects and the mechanisms of autophagy on As_2O_3-induced death of HL60 cells.  METHODS After treatment with As_2O_3 , the growth inhibition of HL60 cells was assessed by MTT colorimetric assay. MDC staining and Transmission Electron Microscope (TEM) were used to examine autophagy induced by As_2O_3 in HL60 cells. To examine the involvement of autophagy in As_2O_3-induced death of HL60 cells, the autophagy specific inhibitor 3-methyadenine (3-MA) was added with As_2O_3 and the cytotoxicity of As_2O_3 was measured by lactose dehydrogenase (LDH) leakage. Immunohistochemistry, flow cytometry and Western blot analysis were used to study the apoptotic and autophagic mechanisms involved in death of HL60 cells.  RESULTS The proliferation of HL60 cells was significantly inhibited in a dose- and time-dependent manner after As_2O_3 treatment. Autophagy was induced in HL60 cells as detected by both MDC staining and TEM; The autophagy specific inhibitor 3-MA exerted opposite effects on As_2O_3-induced death of HL60 cells: 3-MA potentiated As_2O_3’s cytotoxicitiy in HL60 cells when administered 1h before As_2O_3; while it attenuated As_2O_3-induced death of HL60 cells when administered 30 min after As_2O_3. What’s more, once autophagy was induced by As_2O_3, LC3, the autophagic protein in mammalian cells was also activated. The expression of cathepsin B, D and Bid increased, mitochondrial membrane potential collapsed, cytochrome c release was induced and caspase-3 activation accelerated during death of HL60 cells following treatment with As_2O_3. Moreover, pretreatment with 3-MA prior to As_2O_3 amplified above-mentioned apoptotic signaling pathways, but it inhibited these signaling pathways when 3-MA was administered post- As_2O_3 insult.