This study evaluates the expressions of Estrogen Receptor (ER), Progesterone Receptor (PR) and Ki-67 antibodies in canine mammary tumours and their prognostic values in regard to their histomorphologic subtypes. For this purpose, 60 tissue samples suspected as canine mammary tumour were examined and classified according to World Health Organisation's (WHO) criteria. Samples were labelled with anti ER, PR and Ki-67 using immunohistochemistry while ER, and PR values were showed significantly higher in benign tumours (P<0.001, P<0.001, respectively), contraversely Ki-67 value was significantly greater in malignant tumours (P<0.001). The statistical difference between malignant tumour types according to ER, PR and Ki-67 expressions were significant (P<0.001, P<0.01, P<0.001, respectively). Among malignant tumours, solid carcinomas and spindle cell carcinomas had the highest Ki-67 and lowest ER and PR expressions, complex adenocarcinomas and carsinosarcomas had the lowest Ki-67 and highest ER and PR expressions. It is concluded that presentation of immunohistochemical expression of differences between ER, PR and Ki-67 in canine mammary tumours, will provide contribution to the evaluation of tumours and determination of treatment processes in veterinary clinical pathology.