The paper presents analysis and design of a Q-band subharmonic mixer (SHM) with high conversion gain. The SHM consists of a local oscillator (LO) frequency doubler, RF pre-amplifier, and single-ended mixer. The SHM has been fabricated in a high-speed InP double heterojunction bipolar transistor (DHBT) technology using coplanar waveguide structures. To the best of our knowledge, this is the first demonstration of an SHM using InP DHBT technology at millimeter-wave frequencies. The measured results demonstrate a conversion gain of 10.3 dB at 45 GHz with an LO power of only 1 mW. The fundamental mixing product is suppressed by more than 24 dB and the output is around . The mixer is broadband with a conversion gain above 7 dB from 40 to 50 GHz. The conversion gain for the fabricated SHM is believed to be among the best ever reported for millimeter-wave SHMs.