Fludarabine and busulfan becomes standard conditioning regimen for adult patients with acute myeloid leukemia (AML) or myelodysplasia syndrome (MDS). The overall relapse rate is 15\ 20%. More recently, the investigators demonstrated that conditioning regimen with dual alkylating agents consistent of fludarabine, busulfan and melphalan achieved a low incidence of relapse (\<10%). This multiple-center randomize study is aim to compare the transplantation outcome in adult patients with AML/MDS undergoing allo-HSCT with conditioning regimen of Flu-Bu4 vs. FLu-Bu-Mel.