To evaluate the effect of the different doses of antithymocyte globulin (ATG) on the incidence of acute GVHD among patients receiving hematopoietic SCT without ex vivo T-cell-depletion from haploidentical donors, 224 patients with standard-risk hematological malignancy were randomized in this study. One hundred and twelve patients received 6 mg/kg ATG, whereas the remaining patients received 10 mg/kg ATG. This study was registered at http://www.chictr.org as No. ChiCTR-TRC-11001761. The incidence of grade III-IV acute GVHD was higher in the ATG-6 group (16.1%, 95% confidence interval (CI), 9.1-23.1%) than in the ATG-10 group (4.5%, CI, 0.7-8.3%, P=0.005, 95% CI for the difference, -19.4% to -3.8%). EBV reactivation occurred more frequently in the ATG-10 group (25.3%, 17.1-33.5%) than in the ATG-6 group (9.6% (4.0-15.2%), P=0.001). The 1-year disease-free survival rates were 84.3% (77.3-91.3%) and 86.0% (79.2-92.8%) for the ATG-6 group and ATG-10 groups, respectively (P=0.88). In conclusion, although 6 mg/kg ATG applied in haploidentical transplantation decreased the risk of EBV reactivation compared with 10 mg/kg ATG, this treatment exposes patients to a higher risk for severe acute GVHD.