A sub-harmonic mixer is presented with micro-strip at 3 mm wave length. The key device of the mixer is a pair of GaAs beam lead anti-parallel Schottky diode (type MS8251). According to the request of the local oscillator (LO), radio frequency (RF) and intermediate frequency (IF) networks of sub-harmonic mixer, the large signal impedance of anti-parallel diode pair, which is stimulated with the LO signal alone, is simulated by harmonic balance method firstly. The LO network is designed based on the result above. Then the small signal impedance of anti-parallel diode pair with LO stimulation is simulated and the RF network is designed. A 3 mm waveguide to micro-strip translation is also designed in the paper, and the sub-harmonic mixer is fabricated on the RF-Duroid 5880 with 0.127 mm thick and dielectric constant εr of 2.22. According to the measurement result, the conversion loss which is lower than 15 dB is achieved with the RF ranged from 90 GHz to 95 GHz and the LO frequency of 46.3 GHz.