The oil impregnated paper capacitor had been produced until 1982, and most capacitors are operated still. Because the operated years pass 20 years or more, and the troubles occurred in some operated capacitors, the research of the deterioration mechanism and development of diagnosing method for the operated capacitor have been continued. In this paper, the chemical composition of the insulating oil and its chemical deterioration under the high electrical field are investigated, and the accelerated life test is carried out by using the miniature capacitor in order to verify the deterioration mechanism. As the result, it is verified that the wax (deterioration product) is generated from the insulating oil by the high electrical field at the edge of electrodes, and the electrical properties of the capacitor are deteriorated by the wax. In addition the breakdown occurred in the damaged part at the insulating paper. These results lead to construct the flow diagram of the deterioration and breakdown process of the oil impregnated paper capacitor.