Numerical analysis of the instantaneous flow and thermal fields was carried out for an in-line type of fin array for the second laminar flow regime characterized by self-sustained flow oscillation. Flow instability occurring in the wake of a fin leads to the production of the scribes of counter-rotating vortices. They accelerate recovery of the wake and are thus effective in enhancing the heat transfer of the downstream fin. Such vortices hit the leading edge of the downstream fin one after another and induce two particular types of elementary fluid motions near the fin surface, which are effective again in the enhancement of the fin heat transfer. Such fluid motions are also found to produce the dissimilarity between the momentum transfer and heat transfer.