In this paper, we argued the possibility of the diagnostic technique for distribution transformers using the top oil temperature rising above the ambient temperature. The proposed diagnostic technique used the reference top oil temperature rising at rated current. We determined the emergency value of the transformer using the limitation of the top oil temperature rising and calculated the loss of life. The top oil temperature rises because of the load currents. In this point, the proposed diagnostic technique was explained. The proposed system measures the load current, top oil temperature and ambient temperature. With the diagnostic device, we tested the top oil temperature rising of t재 transformers. Then the loss of life was calculated by the top oil temperature.