Abstract: The paper introduces a power combiner circuit consisting of four IMPATT-diodes, The rectangular resonant-cavity combining technigue is used very Successfully for millimeter-wave IMPATT diode power combining. Oversized waveguide cavity is used as the resonant-cavity. As a result, the diode spacing in the longitudinal direction is increased to lessen the tequired geometrical size at this very high frequency.The bui Idung block of the combiner is that of a diode mounting structure of cross-coupled coaxial-waveguides. The ends of the cavity are formed by the iris and a sliding short.The cavity is coupled easily with standard waveguides. Mode conversion lossesmay be present due to discontinuities in wavequides.Power combining output having high combination efficiency is obtained experimentally.