【Abstract】Objective To investigate the changes of myoepithelial cells in mammary atypical hyperplasia and breast cancer. MethodsSP immunohistochemistry was used to detect actin expression in normal breast tissue, grade Ⅰ, Ⅱ and Ⅲ atypical hyperplasia and breast cancer. Electromicroscopy was used to observe the changes of ultrastructure of myoepithelial cells. Results Actin was only detected in myoepithelial cells of normal breast tissue and grade Ⅰand Ⅱ atypical hyperplasia. The positive expression rates of actin in grade Ⅲ atypical hyperplasia（70%） and breast cancer（90%） were significantly higher than that in grade Ⅱ atypical hyperplasia（10%），P＜0.01. In mammary atypical hyperplasia, the number of myoepithelial cells increased with disturbed alignment and abnormal ultrastructure. The changes included that the protrusions on the cell surface diminished, myofilaments and pinosomes in the myoepithelial cells of grade Ⅱ, Ⅲ atypical hyperplasia decreased, and the irregularity of the nuclear morphosis and the increase of nuclear heterochromosome were found. ConclusionThe changes of actin expression in atypical hyperplasia are possibly correlated with carcinogenesis of breast cancer, and myoepithelial cells may play a role in carcinogenesis of breast cancer.