Abstract: In this article, an ultra-broadband power amplifier covering W and F bands was developed using the 50 nm GaN HEMT technology of Nanjing Electronic Devices Institute in order to meet the demand for broadband power amplifiers in millimeter wave communications and other applications. First, the experimental data was fitted and extrapolated using the model to get the broadband power impedance of the HEMT. Secondly, the LC structure and high-low impedance microstrip lines were used for broadband matching to design the matching circuits of the output stage, inter-stages and input stage. Finally, the inter-stage matching circuits are comprehensively optimized and the Lange coupler is used for power combining. The measured typical gain and saturated output power of fabricated power amplifier is about 18 dB and 100 mW, respectively, across over 80 GHz~140 GHz range. A power flatness of ±1 dB and high return loss were obtained which indicate broad application prospects in the terahertz fields.