An 85 to 110 GHz high efficiency, high output power frequency doubler is presented in this letter. The traveling-wave topology is used to generate even harmonics and suppress odd harmonics effectively. The output fundamental and 4th harmonic reflectors are applied to enhance the efficiency, conversion gain and fundamental rejection. The measured results show a peak conversion gain of 3.2 dB and peak efficiency of 11.2% with a saturated output power of 9 dBm at 100 GHz. The doubler exhibits higher than 30 dB fundamental rejection from 85 to 110 GHz with a compact chip size of 1.5 0.9 mm .Index Terms-Frequency doubler, high efficiency, high output power, reflectors, traveling-wave.