The processing of SmCo5 sintered magnets involves a post-sintering heat treatment, where the intrinsic coercivity of the magnets may increase more than one order of magnitude. Variables of the heat treatment like time, temperature and cooling rate have strong influence on coercivity. We describe a method for modeling the heat treatment, which includes microstructural features as precipitate size and 2nd phase volume fraction. The numerical solution was obtained using the Finite Volume Method to solve Fick´s second law. Experimental data like the diffusion coefficient of Sm into SmCo5 phase and the Sm-Co phase Diagram are used for the modeling.