A highly compact heat sink has been developed for forced air cooling of LSI packages and heat dissipation and pressure drop performance of the heat sink have been experimentally studied. These heat sinks have a remarkable feature of extremely narrow fin spacing, and hence, large heat-transfer area per unit heat-sink volume. The streamwise fin length was designed to be short to prevent an excessive pressure drop of the air flow. The cooling performance of these heat sinks in forced air flow was measured carefully and compared with that of a typical heat sink commercially used for the cooling of LSI packages. It is shown that the new-type heat sinks have highly improved performance in regard to compactness.