Three dimensional unsteady numerical computations have been made for a model of plate-fin and tube heat exchangers located in a uniform flow field. The numerical results showed a good agreement with experimental data. The structures of the flow and thermal fields in the plate-fin and tube heat exchanger were discussed in detail for the cases of different fin pitches. It was found that the transition of the wake of the tube from steady pattern to unsteady one is more likely to occur as the fin pitch is increased. However, the flow field in the region between the fins still remains steady even in the case of the largest fin pitch. This study also shows several interesting features of the heat transfer from the fin surface. The most interesting one is that the dissimilarity between the momentum transfer and heat transfer is observed in the high heat transfer region in the front of the tube.