The effective field acting on orbital motion electrons, which is relative to the interaction between spin and orbit and so on, is introduced into this paper. The definite expression of magneto-optical Kerr rotation is deduced by means of the classical electromagnetic field theory. It is proved that the real part 0'k and imaginary part 0k" of the polar Kerr rotation θk are directly proportional to the effective field Hi. The quasi-transverse Kerr rotation θkt is directly to Hi2. The temperature characters of θk and θtk depend on that of Hi.