Abstract: In this paper, Copplestone's method is extended to analyse a ferrite loaded cyclic H-plane waveguide junction. Solving and matching in the electromagnetic fields in each region and simplifying the field equation are developed. The internal problem (solving and matching in the e. m. field) of the junction is transformed into a scattering matrix by menas of the eigenvalues of junction. A three-dimentional field theory for the ferrite-loaded cyclic H-plane waveguide junction is thus established. For g=3, the computed values agree well with experimental results.