Relapse of minimal residual disease (MRD) is a major problem after conventional chemotherapy in patients with acute myeloid leukemia (AML). The bone marrow stroma can protect AML cells from insults of chemotherapy, partly contributing to AML relapse. Arsenic trioxide (ATO) is the main component of arsenical traditional Chinese medicines and has been widely used for the treatment of hematologic malignancies particularly acute promyelocytic leukemia over the past three decades. ATO acts through a direct arsenic binding to cysteine residues in zinc fingers located in promyelocytic leukemia protein (PML), thus killing the leukemia stem cells (LSCs). Our prior study has demonstrated that adhesion to stroma cells could render AML cells resistant to ATO but the detailed mechanism remains to be explored. Here, we report that the adhesion-induced resistance to ATO is related to the up-regulation of myeloid cell leukemia-1 (Mcl-1). Homoharringtonine (HHT) can potentiate the anti-leukemia effects of ATO on adhered AML cells by suppressing Mcl-1 through glycogen synthase kinase-3β (GSK3β). Furthermore, a potentiating effect of HHT on ATO was also observed in primary AML cells and AML xenografted tumors. Thus, these data indicate that HHT could enhance ATO anti-leukemia activity both in vitro and in vivo.