A high efficiency 220 GHz frequency doubler based on discrete diodes is presented in this paper. This doubler is realized with a 50-μm-thick, 450-μm-wide, and 2.7-mm-long quartz substrate. The conversion efficiency is better than 16% over the frequency range from 214 to 226 GHz with pumping power of 46.4~164 mW at the indoor temperature. A peak output power of 32mW at 218 GHz with an input power of 161 mW, and several frequency points with efficiency high than 20% are realized. This doubler can act as the pumping stage of the 660 GHz multiplier chain.