The effect of gray surfaces' radiation on mixed convection in a horizontal channel with finite length (aspect ratio A r = L′/H′ = 10) is investigated numerically. The cooling fluid (air, Pr = 0.72) is considered transparent to radiation. The channel is heated discretely from below and insulated from above. Flow patterns, temperature distribution, convective Nusselt number, radiative Nusselt number, and total Nusselt number are presented for different combinations of the governing parameters, which are the surface emissivity ( 0 ≤ ϵ ≤ 1 ), the Reynolds number (3 ≤ Re ≤ 1,000) and the Rayleigh number (104 ≤ Ra ≤ 8 × 105). The results obtained show the existence of two flow regimes controlled by the governing parameters. The multiplicity of solutions is obtained for low values of Re in a limited range of ϵ.