Abstract: In order to obtain high RF output power for the millimeter power amplifier, a new design of waveguide to microstrip transition structure was presented, which was operating by high frequency simulation software HFSS and verified on the module device. The results show that the waveguide to microstrip transition with waveguide dimensions of 2.0mm×7.11mm was fabricated as the RF output structure. Furthermore, the broadband interconnection of the ground coplanar waveguide was used as the new transmission line, by fabricating the impedance matching structure through introducing weld bonding as the matcing type between the microstrip and the probe, which ensures the reliability of the microstrip assembling and the consistency of the RF output. The finial test results covere from 33.5GHz to 355GHz bandwidth in Ka-band, and show good transportation in millimeter band by keeping the stable output from 40dBm to 42dBm. The transition has wide applications in millimeter wave system.