Impinging jet heat transfer is a promising method to cool electronic components. In this paper, a numerical study has been carried out to examine the conjugate heat transfer under a confined impinging jet using a plate-fin heat sink as the target plate. Effects of geometric parameters such as fin number, fin height, and fin-to-spacing ratio are examined over a range of jet Reynolds numbers using dielectric fluid FC-72 as the fluid medium. Thermal resistance, pressure drop, and Nusselt number are the main criteria used to evaluate the thermal and fluid dynamic performance of this flow system. Furthermore, the effects of fin height, fin-to-spacing ratio, and jet Reynolds number on impinging jet heat transfer are obtained. The concept of an effective Nusselt number is introduced for computing the heat transfer effectiveness of heat sinks with different fin numbers.