The characteristic equation for a TE102or TE104rectangular cavity loaded by a ferrite slab in the middle has been derived on the basis of electromagnetic theory. This characteristic equation is used in a new measurement technique with improved precision, theoretical inference, and sample loading. A lot of experimental research and computational analysis have been done for the characteristic equation method. Microwave magnetic spectrum researches have been carried out for a series of polycrystalline microwave ferrites.<UFMMM>