Abstract: A W-Band balanced mixer using microstrip rat-race hybrid-ring is presented in this paper. The radio frequency (RF) signal and local oscillator (LO) signal pass through the antipodal finline waveguide-to-microstrip transitions from the standard waveguide BJ-900 to the microstrip circuit by the isolated ports of the rat-race ring. The intermediate frequency (IF) is combined by jumper and taken to the output port by a high impedance transmission line. The circuit is fabricated on the RT/Duriod5880 substrate with dielectric constant of 2.2, thickness of 0.127mm using two DMK2790 Schottky diodes. When the LO frequency is fixed at 94.5GHz, the conversion loss is less than 14.5dB from 90GHz to 98GHz.