Abstract: Circulating cell- free DNA (cfDNA), which is released by normal cells and cancer cells, is defined as extracellular DNA in the blood. The cfDNA levels in breast cancer patients are higher than those in healthy control donors. cfDNA also carries the features of tumor tissue, such as mutations, methylations, copy number changes, and loss of heterozygosis. cfDNA is a potential bio -marker in the diagnosis, management, and prognosis of breast cancer. In this review, the authors briefly describe the biological features of cfDNA, and discuss its clinical utility as a blood biomarker in quantitative and qualitative research.