Both parallel and staggered plate fin arrays have shown promise for use in high performance heatsinks regard of its individual manufacturing costs. The geometrical and operational parameters are very important to their cooling performance as heatsinks in practical applications. Fluent 5.0 commercial CFD (computational fluid dynamic) code is used to simulate the flow and heat transfer of those heatsinks of different realistic parameters. Based on those simulations, two correlations, concerning Nusselt number and friction factor as the functions of geometrical and operational parameters, FB (fin-base area ratio), PR (ratio of spanwise pitch to lengthwise pitch) and Re, were developed. From the both, the performance comparisons for optimizing geometrical and operational parameters of a fixed dimension heatsink are shown at constant pumping power and constant thermal resistance. Several optimized parameters were obtained with the discussion to various goals in real application. It demonstrates that in some particular situations, the parallel plate fin heatsinks can out perform the staggered ones.