The purpose of this study is to compare the incidences of GVHD and viral infections in haploidentical hematopoietic stem cell transplant recipients receiving different dose of antithymocyte globulin (ATG) for acute graft-versus-host disease(aGVHD) prophylaxis. Our first objective was to investigate the optimal dose of ATG for aGVHD and second object was to evaluate the effect of different dose of ATG on post-transplant viral infection.