Our modern life depends on electric power very much and, this electric power is to be sent by power cable. In order to send the power in stable condition, the reliability of the cable is extremely important. However, for the cables are being used at sea and underground, the degradation phenomena of the cable can be found. Water tree is one of the main degradation of the cables. There are several methods to investigate the degradation of cables. Among them we are using simulation method to detect water tree by using loss current method.