The application of allogeneic hematopoietic stem cell transplantation (HSCT) has been limited to younger patients without organ dysfunctions due to transplant-related toxicities. Recently, non-myeloablative stem cell transplantation(NST) or reduced-intensity stem cell transplantation(RIST) has been developed as a less toxic HSCT, which enables the application of HSCT to patients of advanced age or with organ dysfunction by the use of mild conditioning regimen. The anti-leukemia effect mainly depends on the graft-versus-leukemia (GVL) effect. Several studies showed promising results of NST/RIST for acute myeloblastic leukemia(AML) and myelodysplastic syndrome(MDS). However, this novel treatment is still very toxic compared to conventional chemotherapy. We should continue clinical trials of NST/RIST to evaluate its efficacy and toxicity in patients with AML/MDS.