The oxidative degradation characteristics of a tape lapped joint (following TJ) were examined. Permeability constant, diffusion constant and consumption speed of Ethylene-Propylene rubber tape (following EPR tape), semi-conducting tape and cross-linked polyethylene insulations were measured. A certain amount of oxygen existed in the aperture of a conductor and a wire shield of a cross-linked polyethylene insulated cable. The simulation of the diffusion equation of oxygen in the TJ, which added the oxygen consumption clause, was carried out. It was shown clearly that this oxygen might oxidize locally and cut the EPR tape, which was the insulator of the TJ, by the simulation. The part imitation models of the TJ, which could supply air or oxygen only from a conductor side, were manufactured. These models were heated under constant temperature. The EPR insulation layer fractured from the inner side, and resulted in the whole destruction. Moreover, when the temperature of 105 degrees and operation voltage were added to the used TJ for 2, 877 hours, the insulation of the TJ was destroyed. Only EPR tape fractured over the length of 40cm, and the internal semi-conducting tape was not fractured. We presumed from the simulation result and the destructive aspect, insulation destruction of the TJ was carried out by oxidative degradation of EPR tape. When a high temperature operation of the TJ was carried out, it was shown that there was a possibility of the insulation destruction by oxidative degradation.