Objective To explore the significance and expression of minichromosome maintenance 3 (MCM3) in breast cancer tissue and para-carcinoma tissue, thus to analyse the correlation between the expression level of MCM3 and several clinical factors. Methods According to the patient's clinical pathological data of random collection of patients with breast cancer pathology specimens, it can be divided into tissue adjacent to carcinoma tissues and the two groups. Streptavid-in-peroxidase (SP) immunohistochemical method was used in the trial to examine the expression level of MCM3 protein in 40 cases of breast cancer tissue and corresponding peritumoral tissue. Results Significant difference existed in the expression of MCM3 between breast cancer tissue and corresponding peritumoral tissue, and the difference was statistically significant (P=0.000); The expression level of MCM3 in breast cancer tissue was more inclined to + + + (45.0%), while the expression level was more inclined to -(82.5%) in peritumoral tissue; and the expression level of MCM3 was significantly associated with age (P=0.003), tumor size (P=0.029) and status of axillary lymph nodes (P=0.021), while it was inversely with the status of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor-2 (Her-2) and proliferation cell nuclear antigen (Ki-67). Conclusion MCM3 may play an important role as oncogene in the occurrence and progress of breast cancer. Key words: Breast cancer; Minichromosome maintenance 3 gene; Peritumoral tissues; Immunohistochemistry