In this paper, the effect of radiation heat transfer on mixed convection in a lid-driven trapezoidal cavity is studied numerically. The governing equations of mixed convection are solved based on the SIMPLE algorithm and the solution of the radiative transfer equation inside the participating medium is carried out using the discrete ordinates method, simultaneously. To study the heat transfer and flow characteristics, sensitivity analysis is carried out based on the two parameters including the optical thickness and radiation-conduction parameter. Several interesting results are obtained such as sweep behavior on the isotherms, streamlines and convective Nusselt number with optical thicknesses.