In order to investigate the effect and reliability coming up to properties, of the matter due to the change of solid structure in dielectrics, the effect of dielectric characteristics for thermal treated LDPE film was made researches. Specimens of LDPE with thickness 100[㎛] were investigated into the change of solid structure by ageing. Thermal treated specimen were made, that were after applying heat at 100[℃] for 1[hour] ① air-cooled specimen slowly, ② water-cooled specimen under the room temperature, liquid nitrogen gas-cooled specimen rapidly. With specimen of thermal treated three types turn out and original, it was for dielectric characteristics to be experimented in the temperature range of 20∼120[℃], frequency range of 30∼1.5 × 10^5 [Hz], appling voltage from 300 to 1500 [mV]. Consequently, the degree of crystallinity was changed with 49∼57[%] according to the thermal treatment. In case of frequency, 100[㎐], on the thermal dependance in dielectric characteristics, tan decreases due to cooling method.