Electromagnetic wave is used for detectin of partial discharges(PD) in GIS(Gas Insulated Substation). Very wide frequency band up to 1. 5GHz is utilized for the detection. There are three categories of electromagnetic wave which can be excited and propagate in coaxial cylindrical structure like GIS: TEM, TE, TM modes. The characterization and identification of each mode are indispensable to understand the partial discharge phenomena in GIS. In this paper, peaks in frequency spectrum excited by PD are identified in its mode and the resonant characteristics of each mode are studied. The followings are the major results: (1) The resonant peaks appear at the frequencies defined by the spacer distance. (2)The intensity of each mode depends on the position of discharge source along the radial direction. The lower order modes including TEM are excited when a discharge source is located near the center conductor. On the other hand, higer order modes are excited being the discharge source at the enclosure surface. (3) The experimental profile of each mode excited by PD along radial direction approximately agrees with the general theory of electromagnetic wave propagation in coaxial cylindrical cavity.