This paper presents the optimal number of sensor and placement in 170 kV (13.8 m × 3.2 m × 4.8 m and 362 kV (15.6 m times 14.5 m times 8.5 m}) GIS based on finite difference time domain (FDTD) method. The 5 pC discharge using artificial defect was measured according to international council on large electric systems (CIGRE) TB 654. The experiment environment according to CIGRE was simulated to analyze attenuation of electromagnetic (EM) wave in prototype GIS. The reference attenuation ratio (RAR) to detect 5 pC PD was analyzed depending on the distance between PD source and sensor in prototype GIS using FDTD method. The optimal sensor placement was proposed in 170 kV, 362 kV GIS based on RAR. In addition, the channel characteristics of GIS was analyzed and proposed for the target frequency band of the ultra-high frequency (UHF) sensor.