The outdoor polymer insulating materials are exposed to UV-ray, one of the most severe ageing factor for the polymeric materials, in natural environment. Therefore, it is suggested that UV-ray induces the surface chemical structural change with the long-term outdoor use, which makes their electrical insulating performance such as the tracking resistance decrease. After the UV-ray irradiation to samples, the tracking resistance, surface hydrophobicity, surface appearance, microscopic surface appearance and surface chemical structure are compared with these of the original. EVA, Silicone Rubber, Modified Silicone Rubber, Flame-resisting Modified Silicone Rubber, Epoxy and Silicone Resin Coating are investigated. The experimental results show that the tracking resistance decreases and/or increases due to the UV-ray irradiation. From the results of surface chemical analysis by FT-IR spectrometer and X-ray diffraction analyzer, the possible chemical reactions induced by UV-ray are considered. Then, it is found that the decrease in the tracking resistance results from the surface chemical structural change of the base polymer. The recovery of the tracking resistance is attributed to the remarkable loss of the surface hydrophobicity, which concerns the contaminant flow form and discharge behavior on the surface.