A theoretical and experimental study was carried out on the thermal performance of a pin fin array heat sink. A theoretical model was utilized based on the success of prior research that has the capability of predicting the influence of various geometrical, thermal, and flow parameters on the effective thermal resistance of the heat sink. An experimental investigation was carried out for measuring the thermal performance of the heat sink, and the overall convective heat transfer coefficient for the fin bundle, including the thermal and flow bundle effect. Utilizing the new empirical correlations, theoretical predictions were made and experimentally validated for a wide range of parameters for combined forced and natural convection in the assisting flow configuration. Both the theoretical model and experimental data indicated the existence of optimal fin spacing.