Abstract: Energy gap is one of the most important physical parameters in Si transisitors. Based on the temperature dependence of the part of the reverse diffusion current in the emitter and the forward voltage of the collector-base PN junction, a new method for the determination of energy gap in regions of the Si transistor is presented in this paper. Because of the heavy doping effect, the carrier's Femi-Dirac statistical distribution is considered. The method only requires measurement of the temperature dependence of the DO parameter; therefore the result is accurate.