Forty five biopsy or mastectomy specimens were collected from dogs for histopathological evaluation of neoplasia and classified according to the new WHO classification of tumors of the mammary gland of the dog. Based on the results, 21 cases of benign tumors were classified in this study into adenoma ‘‘simple or complex’’ (4cases) and benign mixed tumors (5cases) which subclassified into ‘‘fibrolipoadenoma, Chondroadenoma, Osteoadenoma and Chondro-osteoadenoma’’. Meanwhile, 24 cases were diagnosed as malignant tumors, classified into simple adenocarcinoma (4 cases), complex adenocarcinoma (3 cases), solid carcinoma (5 cases), scirrhous carcinoma (1 case), ductular carcinoma (3 cases), mucinous carcinoma (1 case), adenocarcinomas in mixed tumors (4 cases) and carcinosarcoma (3 cases). Out of all detected malignant tumors, 11 cases only revealed metastatic foci in regional lymph nodes; ductular and solid carcinomas had a higher rate of metastasis. In conclusion, recognition of the characteristic morphologic features of ductular carcinomas as a distinct biological entity will be of value in assisting the pathologist and oncologist in evaluating mammary carcinoma prognosis. Moreover, the metastasis rates for the true carcinosarcomas were far in excess of the adenocarcinomas in mixed tumors, emphasizing the importance to separate these two classes of tumors and not lumping them under the classification of malignant mixed tumor.