Three-dimensional CFD models of circular pin fin heat sinksin bypass flow have been developed. The numerical modelshave been validated against experimental data presented earlierby the authors. A parametric study has been performed wherethe inlet velocity, the fin height, the fin-to-fin distance and thepin orientation was varied. For all cases, the duct flow area washeld constant. The flow was turbulent, the Reynolds numbervaried between 6 100 and 15 300. For turbulence modeling theChen-Kim k-ε model was used. Results are presented for theheat transfer coefficient, and the pressure drop, as well as theinfluence of flow bypass, and the leakage from the interfinregion to the bypass regions of the domain. The inlet velocity,the fin height, and the fin-to-fin distance all showed a stronginfluence on the heat transfer coefficient and the pressure drop.