Objective To observe the changes of basement membrane (BM) during carcinogenic change of the atypical hyperplasia of the mammary. MethodsSP immunohistochemical method and two special stain method (Foot and PAS stain method) and the electron microscope were used to observe the changes of the BM. Results BM already changed in atypical hyperplasia grade Ⅱ, and more significantly changed in atypical hyperplasia grade Ⅲ. BM was thinner or thicker somewhere distinctively under the microscope, and some little deletions were observed under the electron microscope in atypical hyperplasia grade Ⅲ, and there was no BM in breast cancer.Conclusion The change procedure of BM was correlated with the changes of the epithelial, atypical hyperplasia. The changes of BM in the atypical hyperplasia are a part of the carcinogenic procedure from the epithelial atypical hyperplasia of the mammary.