A simplified numerical model for the solid state phase transformation from Al(6)(Mn,Fe) to alpha-Al(Mn,Fe)Si phase in 3xxx alloys has been constructed. In this model, the phase transformation is assumed to be initiated by the heterogeneous nucleation of alpha-Al(Mn,Fe)Si dispersoids at the interface between Al(6)(Mn,Fe) particle and matrix and the growth of the alpha-Al(Mn,Fe)Si phase into the Al(6)(Mn,Fe) particle is controlled by the diffusion of Si from the matrix. The model has been implemented into a numerical homogenization model. The simulation results show that the implementation of the phase transformation model improves much the prediction results of the homogenization model on the evolution of solid solution level of alloying elements and the volume fraction evolution of dispersoids in 3xxx alloys during homogenization.