Most of the International Prognostic Scoring System (IPSS) high-risk patients with myelodysplastic syndrome partly responded to hypomethylating therapy even with transient remission， while arsenic trioxide (ATO) had partial effect in patients with MDS. Therefore， we sought to investigate the effects and possible mechanisms of the combination of ATO and decitabine (DAC) in MDS cells. In our study， the MUTZ-1 and SKM-1 cells were treated with ATO， DAC or both. Cell viability， cell apoptosis， levels of reactive oxygen species (ROS) and expressions of the endoplasmicreticulum (ER) stress-associated genes and proteins were examined. Results showed the combination of ATO and DAC synergistically inhibited the proliferation and induced apoptosis of MDS cells. Through the RNA-sequence and GSEA gene function analysis， ER stress-related pathway played an important role in apoptosis of MDS cells induced by the combination of ATO and DAC. ER stress-related genes DNA damage inducible transcript 3， GRP78， and activating transcription factor-6 were significantly highly expressed in combination group than those in single agent groups; proteins were confirmed by western blot. The levels of ROS significantly increased in the combination group. Furthermore， the apoptosis of (ATO+DAC) group MDS cells could be partially reversed by antioxidant agent N-acetylcysteine， accompanied by decreased expression of intracellular ROS and ER stress-related genes. These results suggested that the combination of ATO and DAC synergistically induced the apoptosis of MDS cells by increased ROS-related ER stress in MDS cells.