A numerical simulation is completed to analyze the ignition transient in a solid rocket motor with rear ignition system. Time-dependent compressible SIMPLER algorithm for gas phase governing equations and finte volume method (FVM) for radiative transfer equation are employed, respectively. Two dimensional conduction equation for propellant grain is combined as well. While the standard k - epsilon model with wall function is used for turbulence, the eddy break-up (EBU) model is used for gas phase reaction rate. A detailed development of unsteady flow and temperature fields is obtained. Especially, the radiation is found to play a significant role in igniting the solid propellant by promoting the heat feedback to propellant surface. When the radiation transport was not considered, the predicted ignition and flame spreading sequence was in error, based on a limited experimental data.