To explore the expression level of has_circ_0000615 in peripheral blood samples and evaluate its diagnostic value for breast cancer patients.The peripheral blood samples of 95 breast cancer patients who underwent curative surgical resection and 95 age-matched healthy volunteers in our institutions from September 2019 to November 2020 were systematically collected. The expression level of has_circ_0000615 in the plasma was amplified and detected by qRT-PCR, and its correlation to clinicopathological characteristics of breast cancer patients were analyzed.Breast cancer patients had a significantly higher expression level of has_circ_0000615 in the plasma than healthy controls (P < 0.01), and its high expression was closely associated with advanced tumor stage (P=0.010), lymph node metastasis (P = 0.001) and high grade of recurrence risk (P=0.012). The receiver operator characteristic (ROC) curves showed that the area under curve (AUC) value, sensitivity and specificity of has_circ_0000615 for the diagnosis of non-metastatic breast cancer was 0.904 (95% CI: 0.863-0.944), 76.8% and 88.4%, respectively. Serum has_circ_0000615 expression had a better diagnostic efficiency than routine tumor biomarkers such as CA153, CA125 and CEA for distinguishing breast cancer patients from healthy individuals. TEM revealed that isolated exosomes from the culture medium of breast cancer cells had a disk-like appearance with a diameter of 80-200 nm vesicles, and the expression of exosome markers CD9 and CD81 was markedly increased. More importantly, the expression of has_circ_0000615 was detected in the exosomes and its expression level was markedly upregulated in breast cancer cell lines compared with normal ductal epithelial cells. The stability assay showed that there was no difference between RNA extraction at 0 hour and 24 hours in terms of the expression of has_circ_0000615 (P =0.327). Has_circ_0000615 might as exosomes be secreted into the circulating blood of breast cancer patients, resulting in a high expression level in plasma samples.The detection of has_circ_0000615 might be a promising diagnostic method for breast cancer.© 2021 Liu et al.