The current clinical diagnosis and treatment of breast cancer relies on imaging and several prognostic/predictive biomarkers such as estrogen receptor, progesterone receptor, and HER2. Since these biomarkers are mainly derived from primary tumor tissues, they are limited to be applied for monitoring metastases and recurrence, especially after the removal of primary lesion. The discovery of circulating cell-free microRNAs (circulating cf-miRNAs, also called circulating miRNAs) may offer the opportunity to improve the clinical cure patterns of breast cancer. The mechanism of active excretion of cell-free miRNAs via exosomes, microvesicles and other transporters may play an important role in the formation of circulating miRNAs. Cell-free miRNAs, especially circulating miRNAs, not only act as endogenous signaling molecules to influence behaviors of tumor cells and their microenvironment, but also regulate invasion and metastases of breast cancer cells by communication with other signaling pathways to regulate tumor angiogenesis and epithelial-to-mesenchymal phenotypic transition of cancer cells. This article reviews the molecular mechanism of active excretion of circulating miRNAs and the clinical potential of breast cancer-related circulating miRNAs as liquid biopsy biomarkers in diagnosis, prognosis and response evaluation of breast cancer.