This paper examines the electrical properties according to a curing time and a corona degradation in epoxy composites which are used for transformers. To consider these phenomena, the electrets were formed by appling high voltages to five kinds of specimens designed according to a constant mixing rate and then TSC(Thermally Stimulated Current) values at the temperature range of -160-200[>$^{circ}C$] were measured from a series of experiments. The behaviour of carrier and its possible origins in epoxy composites were studied. Various effects of curing time and electric field on epoxy composites were also investigated.