To improve the flow and heat transfer performance of water flowing through flat rectangular pin fin cooling ducts for electric vehicle power modules, the multi-objective optimization of staggered arrays of streamlined pin fins has been conducted with CFD simulations and multi-objective optimization software. The base model of the streamlined pin fin in the optimization was constructed by adopting a symmetric four-digit NACA airfoil. The constructed model was then used for multi-objective optimization to maximize the heat transfer rate while minimizing the pressure drop in the arrays of pin fins. In the flow visualization experiments, a two-dimensional PIV analysis using the direct cross-correlation method was introduced to evaluate velocity vectors around pin fins. The visualization results with PIV analysis were used to validate the CFD simulation. The multi-objective optimization was performed with the validated CFD simulation and provided Pareto optimal solutions. The optimized staggered arrays of streamlined pin fins have better performance of flow and heat transfer than those of circular pin fins in the low-pressure drop region.