An expression for the average power-handling capability (a.p.h.c.) of microstrip lines is derived by considering the heat generated in the strip conductor and the dielectric substrate. The analogy between electrostatic fields and heat-flow fields is used for calculation of the rise in temperature of the strip conductor. The computed value of a.p.h.c. for a 50 Ω line on alumina substrate (0.06 cm [25 mil] thick) is 5.26 kW at 10 GHz whereas the corresponding figure for polystyrene substrate is 126 W.