A 190~225GHz high multiplying efficiency frequency doubler was developed with discrete GaAs planar Schottky diode. The 50μm thick quartz circuit substrate is flip-chip mounted for diode thermal dissipation, as well as RF signals and DC grounding effectively. Diode embedding impedances were calculated by full-wave analysis with lumped port to represent the nonlinear junction for circuit matching. The doubler is self-biasing and fix-tuned. The highest efficiency of 9.6% and corresponding output power of 8.25mW were obtained at 202GHz with pumping power of 85.5mW. The typical tested efficiency is 7.5% in 190~225GHz. The multiplying efficiency features flat and broadband operation. The doubler reaches the state-of-the-art performance reported worldwide.