A numerical study of forced convection in a horizontal parallel-plate channel with a transverse fin located at lower channel wall is investigated. Through the use of a stream function vorticity transformation, solution of the transformed governing equations for the system is obtained using the control-volume method with non-uniform grid. The effects of the Reynolds number, thermal conductivity ratio of fin to fluid and fin profile area on heat transfer rate of the fin are presented. The results indicate that the optimum aspect ratio of a fin corresponding to the fin with maximum heat transfer rate increases with increasing Re but decreases with K for a fixed fin profile area. In addition, the optimum aspect ratio of a fin obtained from this study for smaller fin profile area and Re tends to approach that of analytical solution assuming constant heat-transfer coefficient. Finally, good agreements are found between the numerical predictions of this study and other experimental data. (C) 2010 Elsevier Ltd. All rights reserved.