This study aimed at establishing aspects of comparison between canine and human mammary gland tumors. In order to achieve that, 44 specimens of canine mammary tumors and 70 specimens of human breast tumors were examined in order to state points of similarity in morphology, cells of origin, behavior and existence of tissue markers. We found that the ratio of benign to malignant forms was 4.5% to 95.5% in canine samples and 45.7% to 54.3% in human samples. In both species, malignant tumors composed of infiltrating ductal cells were in the forefront despite dissimilarity in nomenclature. Other kinds of lower rate of existence included, invasive micro-papillary carcinoma, mucinous carcinoma and invasive comedo-carcinoma which have displayed similar morphology and terminology in both species. Chondroid and squamous differentiation were found in human and canine specimens. In human samples, fibro-adenoma mounted up to 30 out of 32 benign tumors (93.8%). Her-2 and Ki-67 were detected in the specimens of both species but ER was detected in human specimens only. Our findings suggest a significant degree of resemblance between certain mammary tumor in canine and human. Such findings are in support of employing dogs in research investigation for human mammary tumors.