Power Electronic components require cooling to limit junction temperatures to ensure long and reliable operation. Heat sinks are used for cooling the components. In applications which entail operation at altitudes, derating factors are provided by manufacturers of heat sink. A conservative design approach will be to apply the derating factor and size the heat sink larger and so heavier than what it needs to be for sea level operation. Heat is lost from the heat sink surface by two modes of heat transfer: radiation and convection. Convection, in turn, can be natural convection or forced convection. The present study aims to determine the effect of altitude on cooling of heat sink on which power electronic components such as power mosfets are mounted.