Second-law analysis on a pin-fin array under crossflow was conducted, from which the entropy generation rate was evaluated. Increase in the crossflow fluid velocity would enhance the heat transfer rate and hence, reduce the heat transfer irreversibility. Nevertheless, owing to the simultaneous increase in drag force exerting on the fin bodies, the hydrodynamic irreversibility increases as well. An optimal Reynolds number thereby exists over wide operating conditions. Optimal design/operational conditions were searched for on the basis of entropy generation minimized. Comparisons between the staggered and the in-line pin-fin alignments were made in this report.