Numerical analysis is performed on the cooling system of a motor-speed controller (inverter). Heat is generated from several electronic components placed on a couple of PCB's in a rectangular cubic box of inverter. The cooling system is designed as regards the materials selection of the inverter frame, the shape and numbers of cooling fins, the determination of cooling fan's characteristics among others. This numerical analysis is carried via a commercial software FLOTHERM from FLOMERICS, Inc. Results are examined for critical and allowable operating range of temperatures for key components.