Arsenic trioxide (ATO) as an anti-tumor drug could induce differentiation and apoptosis in tumor cells.Mesenchymal stem cells (MSCs) play important roles in the hematogenesis of bone marrow. Many reports have shown that the disorder of MSC adipogenic and osteogenic differentiation occurs in some diseases. However,reports about the effects of ATO on MSCs are limited. In this study, we found that 1μM ATO promoted MSC senescence mainly through p21, although it had no effect on apoptosis at this dose. Furthermore, ATO promoted adipogenic differentiation, but inhibited osteogenic differentiation in MSCs. Our study also showed that CCAAT/enhancer-binding protein alpha C/EBPα and peroxisome proliferator-activated receptor gamma PPARγ might be involved in the regulation of adipogenic and osteogenic differentiation induced by ATO. Our results indicated that ATO may exert an anti-tumor effect by influencing bone marrow micro-environment. Moreover, it may regulate the adipogenic and osteogenic differentiation of MSCs.