In this work, we studied the effect of thermal radiation on unsteady gravity flow of a power-law fluid with viscous dissipation through a porous medium. The modified Darcy's law is considered together with the equation of energy transfer in such media. It is assumed that the viscosity is temperature-dependent. We transformed the governing partial differential equations into ordinary differential equations in terms of a suitable similarity variable. We employed Galerkin weighted residual method to solve the resulting non-linear equations. The results show the effects of thermal radiation parameter and other physical parameters involve in the system of flow.