Abstract: A 105 ~108 GHz W-band power amplifier module is designed and fabricated in this paper. The module is composed of microstrip waveguide transition, power MMIC and bias circuit. The process of design and assemble are discussed. The key technology of microstrip waveguide transition is analyzed particularly. The transition is achieved by using E plane probe. The transition model is fabricated and tested after analyzing and optimizing. The insert loss is less than 0. 6 dB and the return loss is less than -10 dB over 100 ~ 110 GHz. The transition has the advantage of low insert loss and wide band. The gain of the power amplifier module is above 13 dB and the output power is above 200 mW. The power amplifier module achieves the expected target.