Abstract: Objective To investigate the expression level of circulating exsomal miR-451a and its significances in therapy monitoring in diffuse large B cell patients. Methods We isolated exsomal RNAs fractions from serum of 56 DLBCL patients before treatment， during treatment and after treatment. The serum of 56 healthy controls was collected at the same time. Quantitative real time polymerase chain reaction （qRT-PCR） were performed to detected the expression level of circulating exsomal miR-451a. Receive operater characteristic (ROC) curve was performed to comfirm the diagnostic efficiency of miR-451a. Chemotherapy effect corresponding with miR-451a was analyzed. Results Circulating exsomal miR-451a was down-expression in DLBCL compared with healthy controls (P<0.000 1)，and the area under the ROC curve (AUC) was 0.737 （95%CI0.645-0.816）. In 43 patients who had complete follow-up information, the patients who obtained remission, including complete remission （CR） and partial remission （PR）, had the levels of circulating exsomal miR-451a gradually increased. While in patients who did not get remission，including stable disease （SD） and progression disease （PD）, had no significant changes of circulating exsomal miR-451a. Conclusion Circulating exsomal miR-451a may be an potential indicator for therapy response monitoring in DLBCL.