Abstract: ［Abstract］Objective　To study the expression characteristics of Let-7 genes in breast cancer. Methods Twenty-eight patients with breast cancer were randomly selected，and their cancer tissue and adjacent normal tissue were collected. TRIzol was used to extract the total RNA and real-time quantitative PCR was used to detect the relative gene expression levels. Results The expression of precursor Let-7 in cancer tissue was（9.65±2.31），which was lower than that in normal tissue（10.05±2.81 ），P = 0.048. Precursor Let-7 had dependence relationship with the long menstruation（b = 0.816，P = 0.029）. The menarche age showed a positive correlation with precursor Let-7 in normal tissue（r = 0.502，P = 0.048）and a negative correlation in cancer tissue（r = -0.484，P = 0.049）. Conclusions The expression of precursor Let-7 in cancer tissue is lower than that in normal tissue. The period of menstruation is a protective factor to breast cancer.