In this paper, an improved design method of millimeter wave wideband transition from double-ridge waveguide to coaxial line is introduced. The proposed design is based on the mode conversion theories and impedance matching techniques. The characteristics of the transition in microwave frequencies are analyzed and simulated by full-wave analysis software. According to the simulated results, a couple of transitions are fabricated. The predicted superduper performances have been confirmed experimentally in a wide frequency range of 18~40 GHz.