Since more than two decades, Partial Discharge (PD) diagnostic technique using UHF sensor has been developed and then widely commercialized in order to monitor the insulation state of Gas Insulated Power apparatus operating at the substation. Regarding the localization of PD source inside GiTr, Trilateration method has been adopted as one of the reliable tools [1]. This method measures time intervals among 3 sensors respectively and then related distances are calculated based on them, which is valid only for two-dimension. In this paper, “Advanced Trilateration method” is developed in order to improve the errors in localization of PD source by using 4 sensors with relevant algorithm.