Objective To investigate the risk factors and outcomes of delayed platelet engraftment（DPE）after unrelated umbilical cord blood transplantation（UCBT）. Methods The clinical data of 318 patients withmalignant hematological diseases treated by UCBT were retrospectively analyzed. According to the time of plateletengraftment，they were divided into the delayed platelet engraftment（DPE）group and the normal（non ⁃ DPE）group for further analysis Univariate and multivariate analysis were used to analyze risk factors of DPE. Coxre⁃gression analysis was used to analyze the effect of DPE on prognosis. Results The incidence of DPE afterUCBT in all patients was 22.01%. The median time of platelet engraftment for the non ⁃DPE and DPE groupswere 93（69，123）days and 32（13，60）days. Allele ⁃level HLA mismatch（< 6/8），lower infused CD34 + cellcount（≤ 1.7 × 105/kg），CMV viremia and grade Ⅱ⁃ⅣaGVHD preplatelet engraftment were high risk factors forDPE after UCBT. The probability of overall survival（OS）and disease free survival（DFS）at 3 years were 32.9%and 74.0%（P < 0.001），32.9% vs. 68.1%（P < 0.001）in the DPE and non⁃DPE group. DPE，weight ≥ 48 kg，and Ⅲ~Ⅳ a GVHD are risk factors that affecting survival rate. Conclusion DPE after UCBT was associated withworse OS. Early identification of risk factors could be beneficial for the managements of complications.