In this work, it has been found for an ellipsoid of a ferrimagnetic with two equivalent sublattices of unlike ions that the frequency spectrum of spin waves consists of two branches and in each of them the dependence of frequency on the wave vector is similar to the spin wave spectrum of a ferromagnetic ellipsoid. Based on this spectrum and the scattering mechanism of spin waves due to the fluctuation of exchange interactions, calculations have been made for the line width of the uniform resonance and that of the exchange resonance. For the former the conclusions obtained by using the model of an equivalent ferromagnetic are still valid; i.e. nonmagnetic impurity ions and the impurity ions whose spins equal to that of the matrix ion on the same sublattice have no effect on the line width of uniform resonance. However, the impurity ions of both these kinds should give rise to the line width of exchange resonance. There exists a large difference in the line width of uniform resonance estimated for various ferrites by making use of our simplified model of ferrimagnetism. Quite probably the fluctuation of the exchange interactions is the main cause of the line width of uniform resonance in a number of ferrites. The exchange resonance line width caused by this mechanism are estimated for ferrites generally of the order of several tens in the oersted.