Limited to inadequate stem-cell doses, cord blood transplantation (UCBT) is accompanied by increased graft failure and delayed haematopoietic recovery. The conditioning regimen is critically important for engraftment, and numerous trials have been undertaken comparing the outcomes of IV Bu and TBI, but there are no comparative data for UCBT. We conducted a retrospective multicentre study to analyse the outcomes of IV Bu and TBI in UCBT patients with haematologic malignancies. Between 1 May, 2008 and 31 Mar, 31 2018, a total of 331 patients from the China Umbilical Cord Blood Transplantation Corporation (IV Bu, n = 131; TBI, n = 200) were evaluated. The cumulative incidence of neutrophil engraftment was 91.6% in the IV Bu/Cy cohort and 98.0% in the Cy/TBI cohort (P < 0.001). The median times to neutrophil engraftment were 16 and 19 days (P < 0.001), respectively. Multivariate analysis showed no statistical difference for nonrelapse mortality (hazard ratio [HR], 1.11; 95% confidence interval [CI], 0.66 to 1.86; P = 0.695), relapse (HR, 0.90; 95% CI, 0.50 to 1.60; P = 0.713) and overall survival (HR, 0.94; 95% CI, 0.61 to 1.44; P = 0.763) between the two conditioning regimens. Our results show that both IV Bu and TBI are valid myeloablative conditioning regimens for haematologic malignancy patients treated with UCBT.