An experimental investigation was conducted on automatic transmission fluid cooling in a minichannel heat exchanger using a closed-loop integrated thermal wind tunnel test facility. Effects of automatic transmission fluid Reynolds number (Re-L) on heat transfer coefficient and Nusselt number were examined within the Re-L of 3-30 for air-flow Re of 1,450-5,200. Effects of serpentine on heat transfer enhancement and flow characteristics were evaluated through Dean number analysis. The analysis of Eckert number and Brinkman number showed a contribution to the viscous heating even for a low ReL in the minichannel. The study showed enhanced heat transfer characterizations of the multi-port minichannel heat exchanger.