Tetra-arsenic tetra-sulfide (As4S4) is an arsenic compound with antitumor activity, especially in acute promyelocytic leukemia (APL) that are resistant to retinoic acid (RA). Although recent studies have revealed that the therapeutic action of As4S4 is closely associated with the induction of cellular apoptosis, the exact molecular mechanism underlying this action in RA-resistant APL remains to be clarified. In this study, we found that As4S4-induced apoptosis was accompanied by reduced mRNA and protein expression of SET gene in RA-resistant NB4-R1 cells. Moreover, RNAi knockdown of SET gene further promoted As4S4-induced apoptosis, while SET overexpression recovered the cell viability, suggesting that As4S4 induces apoptosis through the reduction of SET protein in NB4-R1 cells. We also observed that the knockdown of SET gene resulted in the upregulation of protein phosphatase 2 (PP2A) expression and the downregulation of promyelocytic leukemia and retinoic acid receptor α fusion gene (PML-RARα) expression, which were enhanced by As4S4 treatments. By contrast, overexpression of SET gene resulted in PP2A downregulation and PML-RARα upregulation, which were abolished by As4S4 pretreatment. Since PP2A is a proapoptotic factor and PML-RARα is an antiapoptotic factor, our results suggest that As4S4-induced apoptosis in RA-resistant NB4-R1 cells is through the downregulation of SET protein expression, which, in turn, increases PP2A and reduces PML-RARα expressions to lead to cell apoptosis.