The theory of ferromagnetic ·reson.ance absorption is discussed in Hamiltonian form. The general formula of magnetic resonance absorption in a ferromagnetic single crystal is obtained when the applied magnetic field has arbitrary intensity and direction. This formula reduces to Kittel's formula in the case of infinitely strong applied field. When a magnetic field is applied. in a direction near the hard axis, an additional resonance is possible in the non-aligned state of magnetization vector to the direc· tion of the applied field. It is concluded, however, that the mechanism described above 1s not responsible for double peaks in iron single crystal which are reported by Kip-Arnold and m cobalt ferrite by Tannenwald. The resonance frequencies for antiferromagnetic crystals of ordinary two mblattices type and that of triangular lattice are obtained by a similar method.