To evaluate if additional cord blood transfusion could accelerate the hematopoietic reconstitution in severe aplastic anemia(SAA) patients receiving IST therapy, 120 eligible patients will be randomized to two groups, the IST group and the cord blood transfusion group. Patients in the IST group receive standard IST which including ATG (Thymoglobuline®, Genzyme) 3.5mg/kg/d×5d plus oral cyclosporine A（CSA ) started from 5mg/kg/d and adjusted to maintain trough serum concentration of 200-300ng/ml. While patients in the cord blood transfusion group receive the same dose and course of ATG and CSA as the control group and one unit of cord blood having no more than 2 HLA-A, B and DRB1 mismatches is transfused 24h after last dose of ATG administration.