The effect of thermal radiation on unsteady magnetohydrodynamic free convection flow of an optically thin gray gas past an infinite inclined plate with constant temperature has been investigated. The governing coupled partial differential equations are solved analytically using the Laplace transform technique. The dimensionless velocity and temperature profiles are shown in graphs and the numerical values of the non-dimensional skin-friction and Nusselt number are presented in tables. The influence of the system parameters such as Prandtl number, inclination angle, magnetic field parameter, Grashof number, radiation parameter and time on flow fields have been discussed in detail. The results indicate that the inclination angle, magnetic field parameter and radiation parameter effects were to decrease the fluid velocity along an inclined plate. © (2012) Trans Tech Publications, Switzerland.