Abstract: The coupling properties of a transverse slot in the common wall between butting two rectangular waveguides are discussed. By means of Galerkin s method which takes into account the wall thickness, equivalent magnetic currents on the aperture surface of the slot are obtained, and then general formulas of scattering fields and of .equivalent parameters arc derived. The dimensions as well as seats of those two waveguides and the slot, and the thickness of the wall are arbitrary. For example, the transforming coefficients and the VSWRs of numerical calculation and experiments are given. The theoretical and experimental results are in good agreement.