The tumor microenvironment (TME) is an important component for studying tumor behavior in several cancers in human beings. However, little information regarding the role of the TME in canine mammary tumors (CMTs) is available compared to humans. In this study, the aim was to investigate the relationship between the TME, hypoxia and angiogenesis through CD31, VEGF, HIF-1a, CD68 and CD163 expression by using immunohistochemical (IHC) methods in formalin-fixed paraffin-embedded canine mammary tumor samples [(n=34: malignant (n=28) and benign (n=6)], to compare them with the clinicopathological features of tumors, and to analyze the relationship between them. There was no significant relationship between CD31, VEGF, HIF-1a, CD68 and CD163 expression in malignant tumors compared to benign tumors (P>0.05). There was an association between microvessel density and clinicopathological variables (the tumor size P=0.013, the presence of necrosis P=0.022) and individual histological grade (G2 vs. G3 P=0.028) in malignant tumors. While there was a positive correlation between CD68 and CD163 in malignant tumors in the dogs (P<0.01), no correlation was determined between other antibodies. Immunohistochemical determination of the level of angiogenesis in the TME may give further useful information about the angiogenic potential and grading of the clinical aggressiveness of some CMTs.