Monolithic GaAs impatt diodes were developed as CW power sources at 60 GHz using double-drift flat-profile structures prepared by MBE. Two mesa-type diodes were employed to excite a microstrip resonator produced on the same chip as the diodes. The oscillation frequency was determined by the microstrip dimensions and the diode locations rather than by external circuitry 1.2 W was obtained at 60 GHz with 8.5% efficiency, and a maximum efficiency of 10% was obtained at 58.5 GHz with 1.1 W output power without the use of diamond heat sinks.