The minimization of integrated circuits in powerful electronic devices generates a large amount of heat that has to be removed effectively and efficiently. One of the methods applied is to use a microchannel heat sink. Past optimization attempts have looked at the microchannel geometry, material, and coolant types using various models to represent the heat sink. This paper reports the analytical study on the optimization of the thermal resistance and pressure drop of a rectangular microchannel heat sink using a new coolant, ammonia gas which has never been reported before. The applicability of ammonia gas as a new coolant was investigated and showed excellent performance. The effect of different channel aspect ratio, fin spacing ratio, different structural materials and Reynolds number was investigated. Significant reduction in thermal resistance was obtained with 0.213 oK/W for ammonia gas compared to that of 0.266 oK/W for air under the same operating conditions. The total pressure drop achieved was 4.82 mbar and 9.52 mbar for ammonia and air respectively. The results indicate promising potential for ammonia gas as a coolant for rectangular microchannel heat sink
