This study presents a new model that describes foam displacement in two layers with differing permeabilities, using three-phase fractional flow theory. Results showed that not only frontal velocities but also saturation profiles should be considered to evaluate diversion process. There are two conditions to be satisfied for successful diversion: (1) the mobility reduction factor should be relatively large in both layers to form piston-like displacements; and (2) the mobility reduction factor of the low-permeability layer should be relatively smaller than that of the high-permeability layer for displacement front in the low-permeability layer to catch up with that in the high-permeability layer.