This paper is to detect the expression differences of serum miR-21 in breast cancer, in order to further clarify the function of miR-21 in familial and TNBC pathogenesis of breast cancer. The serum had been collected for healthy check-up females, women with high risk of breast cancer and different types of breast cancer patients. Nematodes were taken as the external reference, real-time fluorescent quantitative PCR detection were taken for expression level of miR-21 in 77 cases of serum. The miR-21 expression level of familial breast cancer group, TNBC group and breast cancer high risk group were significantly higher than that in normal control group and other breast cancer group, P<0.01. Serum miR-21 expression level was associated with lymph node metastasis and Ki67 high expression, P<0.01. Results had proved that serum miR-21 expression quantity increased in familial breast cancer and TNBC and was correlated with lymph node metastasis and Ki67 expression. Serum miR-21 was closed related with TNBC and familial breast cancer. The relative expression quantity of miR-21 in breast cancer serum had no obvious relation with unilateral or bilateral tumor and menstrual situation. Its increased expression might be correlated to the breast cancer hereditary, malignant degree and prognosis judgement and its mechanism required further research.