Studies on a SF_{6}-mixture and -alternative gas has been in progress to reduce the use of SF_{6} gas as an insulation material of GIS (gas insulated switchgears). In this paper, we dealt with PD (partial discharge) characteristics in pure SF_{6} and N_{2}, and their mixtures on aspects of insulation design and risk assessment for GIS. A POC (protrusion on conductor) and a POE (protrusion on enclosure) as the major defects were fabricated to simulate PD. We analyzed the DIV (discharge inception voltage), DEV (discharge extinction voltage), pulse magnitude, counts and phase distribution of PD pulse in SF_{6}-N_{2} mixtures (SF_{6} 100%, SF_{6} 80%-N_{2} 20%, SF_{6} 50%-N_{2} 50%, SF_{6} 20%-N_{2} 80%, and N_{2} 100%) according to the IEC60270. The DIV, DEV as well as magnitude of PD pulse decreased on the POC as increase of N_{2} ratio. For the POE, the DIV and DEV in N_{2} ratio below 50% were the same voltages as those in SF_{6} 100%. In this experiment, SF_{6} 80%-N_{2} 20% mixture could be considered with the equivalent insulation performance to a GIS.