Heat transfer and pressure drop performances are experimentally studied for the flow through wire-fin cores which consist of closely packed thin wire arrays. A modified single-blow transient test method is used to evaluate the performance. The range of the Reynolds number tested is 20-200. Results of the heat transfer and pressure drop are compared with those of the conventional compact heat exchanger surfaces. It is shown that the wire-fin surfaces yield markedly higher heat transfer performance. The flow visualization methods of dye injection and hydrogen-bubble generation are employed to investigate the entire flow field in the cores. Steady laminar flow, vortex shedding, oscillating flow, and turbulent flow are observed in sequence along the flow in the cores.