According to the World Health Organization, one in eight to ten women will develop breast cancer. Therefore, the development of new blood / serum markers is important as an alternative to traditional diagnosis methods. Early detection of breast cancer plays an important role in choosing the right treatment approach and treating the patient. In this study, the expression of miRNA21 gene in the plasma sample of people with breast cancer compared to healthy people was evaluated. The results of qPCR showed that the expression levels of miRNA-21 in the plasma samples of breast cancer patients were significantly increased compared to those of the healthy controls. These results suggest that Mir 21 could be a very good blood biomarker for the early detection of breast cancer.