Heat transfer and pressure loss performances are experimentally studied for the flow through the pin-fin arrays, which consist of closely arranged very thin pins, in order to investigate the effect of turbulence on the heat transfer mechanisms. The flow visualization tests were carried out for enlarged pin array models using a dye injection technique. Heat transfer tests utilizing a modified single blow transient testing method were also conducted. The range of the Reynolds number tested is 20 - 100. The test results showed that the predominant factor of oscillation mechanism in the pin array is the perturbation accumulation from the inlet region. The effect of the oscillating flow on the heat transfer performance has been clarified quantitatively.