Mammary cancer is the most prevalent canine tumor and frequently results in death. This study was carried out for the diagnosis of three canine breast tumors based on clinical, histopathological, immunohistochemistry, and in situ TUNEL assay. Firstly, the morphology of tumor cells was examined by hematoxylin and eosin staining. We found two cases of ductal carcinoma characterized by ductal dilatation and one case of solid carcinoma characterized by extensive connective tissue proliferation. Immunohistochemical assay in paraffin-embedded sections immediately followed surgical removal. The results demonstrated that the expression level of Estrogen receptor (ER), progesterone receptor (PR), p53, vascular endothelial growth factor (VEGF), and proliferating cell nuclear antigen (PCNA) proteins were abnormal. Finally, in situ TUNEL showed a significant decrease in apoptotic cells in canine breast tumors (P<0.01). Taken together, clinical, histopathological, immunohistochemistry, and in situ TUNEL assay will provide a better understanding of the pathogenesis of breast tumors in dogs under field conditions.