A numerical investigation of conjugated forced convection — conduction heat transfer of a plate fin is presented. Laminar and turbulent cases are considered. A second-order finite difference technique and a two-equation turbulence model are applied. Results are presented for various values of the convection-conduction parameter and the Reynolds number. It is found that if the boundary layer flow is mainly laminar or turbulent, the conventional fin theory yields acceptable prediction of the fin efficiency although errors in the fin temperature and local heat flux distributions may occur. However, if the boundary layer has laminar and turbulent parts of about equal spacial extent, an application of the conventional theory may result in large errors even in the efficiency.