Abstract: Objective To investigate the relationship of PCNA expression and MC density in the tissues of breast cancer. Methods Immunohistochemical method and Bismark brown staining were used respectively for the observation of proliferating cell nuclear antigen (PCNA) and mast cell(MC). Results 1.PCNA positive expression were correlation with breast cancer differentiated degree and lymph metastases. High differentiated cancer positive expression were lower than mediated and undifferentiated cancer,P<0.05;PCNA positive expression with lymph metastases were higher than without lymph metastases,P<0.01. 2.As for MC counts PCNA positive cases were lower than PCNA negative ones,PCNA positive expression were negatively correlated with MC density around cancer. Conclusion PCNA were a good prognostic factor of breast cancer,MC gave play to their anti-tumour effects probably through influencing PCNA expression.