Abstract: Objective To investigate the relationship between dynamic monitoring of circulating endothelial cell （CEC） count and chemotherapy efficacy in breast cancer patients. Methods Clinical data of 42 breast cancer patients from May 2013 to May 2015 were collected. The number of CEC in peripheral blood of patients was detected by flow cytometry. The relationship between CEC count and clinicopathological features of breast cancer and the chemotherapy efficacy was analyzed. Results The number of CEC in peripheral blood of 42 breast cancer patients was 339.26±184， and the CEC in 20 healthy volunteers was 85.95±60.47， the difference was statistically significant （P<0.05）. The number of CEC was associated with TNM staging， not associated with age，ER，PR and HER-2. After 2 cycles of chemotherapy， the response rate （RR） in CEC increasing group was 31.6%， and RR of CEC reducing group was 78.4%（P<0.05）. After 4 cycles of chemotherapy， the RR in CEC increasing group was 46.2%，lower than 87.0% of CEC reducing group， and the difference was statistically significant（P<0.05）. Conclusion The dynamic changes of CEC in breast cancer patients receiving chemotherapy was associated with the RR of chemotherapy.