Using a general 3-D finite-volume code, the development of buoyancy force driven secondary flow and its effects on developing laminar flow and heat transfer have been numerically simulated for a horizontal fuel cell duct with a rectangular cross section. The constant thermal-properties assumption except for linear density variation with temperature in the body force term is applied, and a constant heat flux is prescribed on the bottom wall, while thermal insulation is implemented on the other three walls. The secondary flow forms vortices in the duct that can disrupt both the hydrodynamic and thermal boundary layer and enhance friction factor and heat transfer. Calculations have been performed to determine the effects of various Grashof number Gr* and Reynolds number Re. Comparisons of these numerical results with available existing data are presented. This study may be regarded as an improved modeling procedure for gas flow and convective heat transfer in fuel cell ducts.