The present paper is to study of an effect of heat source on mixed convective heat transfer of non-Newtonian fluids through porous medium on a flat plate has been investigated using a modified power – law viscosity model. This model does not contain physically unrealistic limits of zero or infinite viscosity as are encountered in the boundary - layer formulation with traditional models of viscosity for power – law fluids through porous medium with magnetic field. These unrealistic limits can introduce an irremovable singularity at the leading edge; The present modified model matches well with the measurement of viscosity, and does not introduce irremovable singularities. Therefore, the boundary layer equations can be solved by marching from the leading edge downstream as for Newtonian fluids. The numerical results are presented for a shear-thinning fluid in terms of the velocity and temperature distribution, and for important physical properties, namely the wall shear stress and heat transfer rates.