Abstract: A theoretical analysis is given in this paper to the structure of a cross-coupled coaxial-waveguide diode mount--a major module of power combiner. The equivalent circuit isobtained. It is shown by computer simulation that the shorted out position l4 of a cavity can be determined and the power ratio is related to the mismatched load z1, to the output impedance z3 of cavity and their position l1 and l3, to the radius r of inner conductor of a coaxial line and to the distance between coaxial lines. According to these results, a optimum design of millimeter wave combiner is discussed proved by expriments.