Abstract: Objective To explore whether arsenic trioxide(As2O3)-induced apopotosis in drug-resistant leukemia K562/ADM cells is involved in endoplasmic reticulum stress-related apopotosis. Methods The apoptosis of K562/ADM cells was identified by double staining of FITC-Annexin V and propidium iodide (PI), and the morphological ultrastructure of the cells, endoplasmic reticulum and mitochondria was observed using transmission electron microscopy. Flow cytometry (FCM) techenique was employed to assess mitochondrial inner membrane potential(Δψm), intracellular calcium concentration, cytochrome c (Cyt c) release and caspase-3 activity. The expression of GRP78 protein was analyzed by Western blot. Result During the apoptotic process of K562/ADM cells induced with 2 mol/L and 5 mol/L As2O3, the endoplasmic reticulum exhibited obvious expansion and degranulation, and the mitochondria illustrated inner and outer membranes fusion, reduced and confused cristae, swelling and vacuolization. The mitochondrial Δψm decreased, the intracellular calcium concentration and releasing of cytochrome c from mitochondria increased, and caspase-3 was activated. Western blot result indicated upregulation of GRP78 protein at endoplasmic reticulum in apopototic K562/ADM cells. Conclusion As2O3 can initiate the endoplasmic reticulum stress in K562/ADM cells, and induces the drug-resistant cell to apoptosis via endoplasmic reticulum-mitochondrial pathway.