Abstract: We described thermal model of three dimenisiond for a two-layer structure, which used in the electrical measurement of the peak junction tempersture of power transistors. The analytical solutions of this model and flow chart of the calculating program are also given. Using this model, we can correct the electrically mesuured average temperature on the surface of the chip to the peak junction temperature. This modification may be up to 10-30 ℃. Compared the measured thermal field by infrared microradiometer, the measuring error of the electrical peak technique is within 8% whereas the error of the standard electrical technique may be up to 50% on the same conditions.