We have been studying the breakdown characteristics of the low-density polyethylene films (LDPE) caused by the partial discharge in the air gap. The purpose of this study is to clarify the effect of the sample temperature rise on breakdown mechanism of LDPE films under the repetition rectangle wave pulses. The large change appears in the pulse number up to breakdown (N_BD) near 250Hz (4ms) in the N_BD dependence of frequency. From these thermograph results, the temperature rises at the diameter 1mm under the electrode needle show in high frequency region. The sample temperature rise shows a correlation to the decrease of N_BD at a frequency. The surface and center temperature of sample are estimated using the heat equivalent circuit. The sample temperature increases by the heat pulses due to the discharge and it decreases by the heat constant (4ms) of this insulation system. If the repetition period of applied voltage is shorter than the heat constant (4ms), the sample temperature increases and the insulation system leads to breakdown at a few numbers of the applied pulses. From these results of the heat image, there are good agreements between the measured sample temperature rise and the estimation value by the heat equivalent circuit.