Most of electric insulation materials used in power equipments are thermoplastics and thermosetting materials. Typical examples of thermosetting materials are epoxy resin and XLPE which is usually used as cable insulation. Both materials have not only good mechanical, electrical properties but also advantages in handling and productivity. However, these materials are not eco-friendly because they cannot be recycled on disposal. In contrast, thermoplastics have not only a advantage that can be recycled to structural materials, but also there are some disadv-antages on the limitation as finite resources and the environmental issue associated with the Kyoto Protocol. In this paper, PLA(polylactic acid), a kind of bio-plastic, was investigated for eco-friendly insulation system. AC and lightning impulse breakdown strength was measured to confirm the insulation characteristic of PLA. And then tracking resistance test was conducted to find out influences by pollution of specimens. In order to evaluate temperature dependence of breakdown strength of specimens, experiments were carried out at 30, 70, 115 ℃. Aluminum Oxide nano powder was added to PLA to improve the electrical properties. The results of experiments of PLA were compared with the results of experiments PA6(Polyamide-6), PA66 (Polyamide-66) and PC(Polycarbonate). As a result of breakdown test, PLA showed good performance in compared with PA6, PA66, where PC showed the highest breakdown strength. The glass transition temperature and the melting point were measured to analyze the breakdown mechanism. From the anylysis, the breakdown seems to be caused by electriomechanical breakdown mechanism. Tracking resistance of PLA was excellent compared to other specimens. The more nano filler were added in PLA, the more temperature dependence of breakdown strength was decreased. From the experiment results, we found that tracking resistance was improved according to the increase of the content of nano filler.