Numerical computation has been performed to determine the influence of buoyant effects on convective flows with the standard κ-ε and the low-Reynolds-number κ-ε models. Results obtained with both turbulent models are compared with the available experimental data. In this work, Kolmogorov velocity, uε = (vε)1/4 , is introduced instead of shear velocity, uτ = √τw/ρ, to avoid the singularity that appears at the separating and reattaching point for both turbulence models. Turbulent Prandtl numbers were allowed to vary in the low-Reynolds-number κ-ε model to mimic the experimental data. Buoyant effects have been investigated with various Richardson numbers for the backward-facing step flow. Various separation patterns as well as vortex shedding were observed beyond a critical Richardson number. In addition, the required grid configuration for accurate results for the low-Reynolds-number κ-ε model has been discussed for the backward-facing step flows.