In these days, the polymer insulators have been increasingly studied and applied for outdoor insulators because it has superior characteristics than porcelain and glass insulators, which have been used for outdoor insulators. This investigation deals with the degradation characteristics of the outdoor insulation material, silicone rubber and EPDM which was one of the most useful polymer material as outdoor insulators, due to the salt fog and electrical stress.. Two salt fog chambers, which are based in ASTM D 2132, were fabricated for this test. And the treated specimen analyzed with various analytic method. With the result of this study, we could make a further understanding about the degradation mechanism of polymer material..