A numerical study is conducted to investigate the effect of thermal radiation on turbulent flow (upstream of the flame front) inside a subscale planar hybrid rocket motor. The physical model adopted for the study is based on an unsteady two-domain (solid fuel and gaseous oxidizer) concept where both domains are assumed to be two-dimensional. Furthermore, the oxidizer gas flow is assumed to be incompressible and turbulent with boundary layer approximations. The radiative heat transfer is incorporated to the energy equation for the oxidizer using the Rosseland diffusion approximation. Fuel is assumed to be a nontransparent isotropic solid. The two domains are coupled through an energy balance at the interface that includes heat transfer due to radiation, conduction, and ablation. The solution to the governing differential equations of the present model is obtained by first linearizing the equations using Newton linearization method, discretizing them by a fully implicit finite-difference technique, and then solving the resulting set of algebraic equations by a block tridiagonal matrix solver. Finally, the proposed mathematical model is used in a parametric study to determine the effect of various operational parameters, such as flame temperature and oxidizer mass flow rate, on heat transfer from the solid fuel. Results indicate the significance of radiation on turbulent convective heat transfer over an ablating solid.