In this paper, we have designed and fabricated high conversion gain Q-band active sub-harmonic mixers for a receiver of millimeter-wave wireless communication systems. The fabricated active sub-harmonic mixer used second harmonic signals of a low local oscillator (LO) frequency. The fabricated mixer were successfully integrated by using a 0.1 mum GaAs pseudomorphic HEMT's and a coplanar waveguide structure. We showed that the highest conversion gain of 4.8 dB has obtained at a LO frequency of 17.5 GHz and a RF frequency of 40 GHz. Conversion gain from the fabricated sub-harmonic mixer is one of the best reported thus far. The active sub-harmonic mixer also ensure a high degree of isolations, which are -35.8 dB from LO-to-IF and -40.5 dB from LO-to-RF, respectively, at a LO frequency of 17.5 GHz. (C) 2003 Elsevier B.V. All rights reserved.