A numerical investigation has been performed to determine qualitatively the influence of the buoyant effect on the mixed convection. The standard κ-ε model with standard wall functions is used to predict buoyancy flows through a backward-facing step for various Richardson numbers. The eddy-diffusivity concept is used in modeling for the buoyant term. The momentum equation for the velocity field and the energy equation for the temperature field are solved simultaneously because of strong coupling between temperature and velocity. The reattachment lengths for laminar and turbulent flows are calculated and the Nusselt numbers and drag coefficients are examined for each case. Various separation patterns as well as vortex shedding are observed after a critical Richardson number. A numerical solution showed that the buoyant-induced vortex shedding enhanced the heat transfer.