The purpose of this study is to clarify the radiation effects on mixed turbulent convection in a horizontal channel. The present study provides turbulence statistics using direct numerical simulation (DNS) in an optically thin medium. When the radiation effect is considered, the flow structure and the temperature distribution in the channel change with an increase in the optical thickness of the fluid. The radiation effect changes the distributions of the temperature fluctuation intensity and the turbulent heat flux. These radiation effects on mixed convection can be clearly explained by the turbulence statistics obtained from the DNS results. (C) 2012 Elsevier Ltd. All rights reserved.