Abstract： Objective To investigate the efficacy of co-infusing cord blood (CB) as the third party cells on graft versus host disease (allo-GVHD) prophylaxis after unrelated or haploidentical donor allogeneic hematopoietic stem cell transplantation (allo-HSCT).Methods From 2007 to 2011,41 patients receiving unrelated or haploidentical donor allo-HSCT were analyzed retrospectively.Twenty-five patients received one unit of HLA 4/6-6/6 matched CB one day before SCT as CB group,and median MNC dose was (1.64 ± 0.49) × 107/kg.Sixteen cases not receiving CB served as control group.All patients received antithymocyte globulin,cyclosporine,methotrexate,and mycophenolate mofetil as GVHD prophylaxis.The incidence and severity of aGVHD,and treatment-related mortality were compared between two groups.Results The main clinical characteristics in both groups were comparable.The cumulative incidence of aGVHD in CB group and control group was 44.0％ versus 68.8％ respectively (x2 =2.403,P＞0.05).The cumulative incidence of grades Ⅲ to ⅣV aGVHD in CB group and control group was 16.0％ and 37.5％ respectively (x2 =2.445,P＞0.05).The 100-day treatment-related mortality in CB group and control group was 12.0％ and 12.5％ respectively (x2 =0.002,P＞0.05).Conclusion Cord blood as the third party cells might reduce the incidence and severity of aGVHD in unrelated or haploidentical donor HSCT.The efficacy and the mechanism of this strategy need to be further explored by prospective randomized controlled trials.