An 11-year old boxer bitch was presented to the Department of Obstetrics and Gynaecology clinic for routine ovariohysterectomy and coincidentally an ovarial mass was detected which was diagnosed as granulosa cell tumor (GCT) in pathologic examinations. Blood sample was collected for hormonal analysis during this surgery in which estrogen concentration was under the reference range, progesterone concentration was normal. Mammary gland enlargements were also detected on the dog during initial examination with no other sign regarding to vulvar discharge/swelling. One month later following ovariohysterectomy, the dog was presented again for bilateral total mastectomy. Tumoral tissues were diagnosed as complex adenocarcinoma grade II, histopathologically. The possible interaction and promotional effect of GCT on the development of mammary tumors (MTs) is not yet fully understood. Regarding to serum estrogen and progesterone concentrations were not above the reference ranges suggest us GCT did not secrete hormones yet. In conclusion, this case is important as it reports the possible effect of GCT on the development of MTs and suggests not to ignore this disease in a dog with mammary tumours.