Equivalent circuit for the Schottky diode was established based on measurement results of I-V and C-V curves. The design of 0.67THz sub-harmonic mixer included three-dimensional structures and broadband matching circuits. Results show that the 3dB bandwidth of RF is 47GHz with the center frequency at 0.685THz. The double-sideband conversion loss is between 13.1dB and 16dB. The lowest double-sideband noise temperature is 11500K at 685GHz.