Abstract: An array of 8 fourth harmonic mixers is presented with micro-strip at 3mm wave length. The key device of the mixer is a pair of GaAs beam lead opposed Schottky diodes(type MS8251). This paper presents a novel fourth harmonic mixer which consists of opposed diode pair module and diplexer module. Opposed diode pair module and diplexer module are fabricated on the RT/duroid 5880 with 0.127 mm and 0.254mm respectively . The measurement results show that the lowest conversion loss is about 17dB.