Adopting the capacitive coupled matching structure, a Ka-band wideband three-stage power amplifier (PA) MMIC was designed by 0.1μm GaAs pHEMT process in this paper. To facilitate the design of wideband PA, a set of formulas was proposed to analyze this matching structure. This structure achieves wide operating band and high power-added efficiency (PAE) simultaneously in the presented PA. The measured saturation PAE is upon 30% over 32~40 GHz with 20 dBm output power and more than 15.5 dB power gain, which demonstrates the effectiveness of this design skill.