The program package which is applicable to solve problems of heat transfer in transistors or integrated-circuit chips or in their stems has been developed with a three-dimensional lumped network model. The thermal resistances of beam-lead transistors calculated with this program are found to be in good agreement with the observed values. For the better design of beam-lead devices, thermal resistances are evaluated with this program for various geometries of the chips and for the passivation films of 1-micron thick SiO 2 , 2-micron SiO 2 , and 0.2-micron Al 2 O 3 or Si 3 N 4 on 1-micron SiO 2 . The results calculated indicate that the thermal resistance is mainly dependent on the thickness of electroplated Au in the beam-lead structure, and that the heat dissipation is especially sensitive to the distance, which is measured along the beam lead from the chip edge to the nearest end of the joining part of the beam lead to the metallized conductor on the ceramic stem.