Most overhead transmission lines in Korea run over the mountain; however, only little study has been made for the effect of mountain fire on polymeric insulator for transmission lines, though the study is significantly required, Therefore, in this study, the authors observed the deformation of the sheds of the insulator under fire, varying the ignition time using artificial ignition testing equipment, and investigated electrical and mechanical characteristics of the insulator through dry withstand voltage test, impulse flashover test and tensile load test. As the results of the tests, the following conclusions were obtained, First, when the insulator was subjected to the fire, the electrical characteristics were slightly reduced, but there was no change on the mechanical characteristics, Secondly, the sheds and sheath of the insulator were not non-flammable but less-flammable. These two results show the high mechanical stability and durability of the insulator under severe fire condition, though the electrical characteristics can be deteriorated as the time that the insulator is subjected to fire, goes by.