In this paper, a 325GHz Sub-Harmonic mixer with 70GHz bandwidth is presented. It will be used for a ultra wideband remote sensing system. The mixer circuit included planar Schottky diode in a balanced configuration, the passive RF/LO and IF matching networks, filtering circuits, as well as transitions from waveguide to suspended micro-strip. Simulated results for the 325GHz mixer achieved DSB conversion loss of 6.4dB. The LO pumped power was 6dBm at 162.5GHz. The conversion loss was less than 8dB from 290GHz to 360GHz.