A high performance U-band monolithic GaAs power amplifier, based on 0.3 mu m gate length MESFET technology, has been developed for millimeter-wave system applications. The MMIC power amplifier, both single-ended and on-chip combined designs, have on-chip DC-block, RF-bypass and bias networks. A four-stage power MMIC chip exhibited 165 mW output power with an associated gain of 14.2 dB at 47 GHz. The chip delivered saturated output power of over 180 mW. This result, to the authors' knowledge, represents the highest power gain and complexity from a single MMIC chip at U band.<<ETX>>