In this paper, the fluid flow and heat transfer characteristics of finned tube heat exchangers were numerically investigated by examining different oval and circular tubes. The three kinds of tubes were applied with two types of enhanced fins, wavy fin and louvered fin. The results reveal that using the oval fin-tube can not only reduce the flow resistance but also improve the heat transfer capacity of the heat exchangers which effectively improved the fin efficiency. As comparing with the big circle-tube louver fin, the heat transfer rate of oval-tube fin is increased by 1.5-4.9%, while the pressure drop loss is decreased by 22.0-31.8%. The fins with the big circular-tube have higher heat transfer coefficients and smaller total heat transfer area. For louver fin, in terms of heat transfer rate, the heat transfer coefficient is more significant than the heat transfer area. It was also found that the parameter of minimum thermal resistance based on the entransy dissipation can be used to optimize the heat transfer performance, and therefore designing the optimal heat exchanger structure. (C) 2013 Elsevier Ltd. All rights reserved.