Big fire such as mountain fire may cause the prevention of the functions of the overhead cables and insulators, which may affect the operation of the transmission lines. In the worst case, this kind of disaster may have a huge effect on the whole industry of a country. However, the study on the effect of the mountain fire on the transmission line is very rare. Therefore, in order to understand the effect of the mountain fire on the polymeric insulator for transmission lines, the author observed the deformation of the sheds of the polymeric insulators and the change of the discs of the porcelain insulators under fire, varying the ignition time using the artificial ignition testing equipment which simulates the mountain fire, and investigated the electrical and mechanical characteristics of the insulators after the ignition test. For the test, the miniature insulators made of polymeric material and porcelain have been utilized. As the result, the following conclusions were obtained. First, the porcelain insulator was degraded in electrical characteristics when the insulator was subjected to the fire for approximately 5 minutes; whereas, the polymeric insulator was not degraded though there were some damage on its sheds. Second, after 20 minute exposure to the fire, the polymeric insulator lost a lot of parts of sheds, but the electrical characteristics was lowered by around $20\%$, but the porcelain insulators were electrically degraded by more than $80\%$.