Background: Aflatoxins are carcinogenic compounds that are produced by Aspergillus flvaus and A. parasiticus fungi growing on foods. Among variety of known aflatoxins, aflatoxin B1 has been reported as the most carcinogenic compounds. This toxin can enter human body through contaminated food. The mammary tissue is one of the body tissues that aflatoxin B1 can penetrate. The BRCA1 and BRCA2 genes are two major effective genes mammary gland tumor suppression, so expression of these genes can be affected by aflatoxin B1. Materials and Methods: In order to evaluate the effects of aflatoxin B1 on BRCA1 and BRCA2 genes expression, normal Human Mammary Epithelial Cells (HMEC) were cultured as monolayer. After 10 days, monolayer cells culture reached suitable amount and was passaged for treatment to 16 flasks 25cm2 (four treatments with four replicates for each one). After proper cells growth, three treatments, 15, 25 and 35 µl of aflatoxin B1 were added to the flasks and then, cells were separated for extraction of RNA, cDNA and Real time PCR performance after 24 hours. MTT assay was performed to measure cell viability. Results: The results showed that all three concentrations decreased the expression of BRCA1 and BRCA2 genes (P<0.0001). In addition, the results of MTT assay showed that cell viability was reduced with increasing concentration of toxin and duration of exposure. Conclusion: According to the results, it seems that aflatoxin B1 can increase the risk of breast cancer by reducing expression of BRCA1 and BRCA2 genes.