Thermal resistance of heterojunction bipolar transistor is usually considered to be a linear function of ambient temperature. However, in power applications, the thermal effects become obvious. To obtain well design characteristics, the thermal resistance cannot only be considered to vary with ambient temperature in linear relation, but also the function of junction temperature. In this paper, the variations of thermal resistance are discussed, and the thermal resistance is almost a linear function of junction temperature. (C) 2003 Elsevier Ltd. All rights reserved.