A wideband MMIC power amplifier at W-band is reported in this letter.The four-stage MMIC,developed using 0.1 μm GaAs pseudomorphic HEMT (PHEMT) technology,demonstrated a flat small signal gain of 12.4± 2 dB with a minimum saturated output power (Psat) of 14.2 dBm from 77 to 100 GHz.The typical Psat is better by 16.3 dBm with a flatness of 0.4 dB and the maximum power added efficiency is 6％ between 77 and 92 GHz.This result shows that the amplifier delivers output power density of about 470 mW/mm with a total gate output periphery of 100 μm.As far as we know,it is nearly the best power density performance ever published from a single ended GaAs-based PHEMT MMIC at this frequency band.