The effects of two different histone deacetylase (HDAC) inhibitors, sodium butyrate (NAB) and trichostatin A (TSA),on apoptosis of human leukemic cells in vitro and the molecular mechanisms were investigated. The experiments were divided up 5 groups: control group, NaB group, TSA group,NaB+Z-VAD-FMK group and TSA+Z-VAD-FMK group. The apoptosis rate was determined by mor-phological analysis and flow cytomytry. The expression of Daxx, Bcl-2, and Bcl-xl proteins was de-tected by West blot. NaB and TSA could induce the apoptosis of HL-60 and K562 cells, and Z-VAD-FMK caused a marked decrease in apoptosis induced by HDAC inhibitors. HDAC inhibitors could down-regulate the expression of Daxx protein, but had no significant influence on the expression of Bcl-2 and Bcl-xl proteins. The results suggested that NaB and TSA induce distinct caspase-dependent apoptosis of human leukemic cells through down-regulating the expression of Daxx protein in vitro.