Eleven years old cocker spaniel referred to Veterianry Medical Teaching Hospital, Seoul National University was diagnosed as pheochromocytoma. The patient was transferred to the hospital after hit by car and the main clinical signs were tachypnea, tachycardia, edema of hindlimb and lethargy. On ultrasonographic examination, hyperechoic foci was detected and it was invaded into the caudal vena cava. Venography showed the filling defect and local dilation of the caudal vena. The diagnosis of pheochromocytoma is challenging due to the nonspecific clinical signs and severe concurrent disease in dogs. This report describes one dog with invasive pheochromocytoma in which radiography, ultrasonography and venacavography were key diagnositc tools. Ultrasonography was a usuful method for assessment of size, shape and margination of the tumor and invasiveness to the major vessels.