Based on the universal additivity law of the interference light an analytic representation for the relation between the enhanced Kerr rotation θk and the intrinsic Kerr rotation θ0k of the magneto-optical materials is obtained theoreticallv for a bila yer films-the difference between the measured herr rotation θk and intrinsic Kerr rotation θ0k is pointed out for the case where an incident fight goes through the glass substrate onto the magneto-optical film, a simple method for measuring the intrinsic of Kerr rotation θ0k is given. 4) the physical reason about the enhancement Kerr ro tation θk by the dielectric medium is discussed, and the reprcsrntation of the enhanced Kerr rotation θk for a bilayer system with plasma absorption edge is deduced theoretically.