Abstract: This paper introduces the mixed theory and design method of a microstrip 4th harmonic mixer worked at Ka band. The mixer mainly consists of a waveguide-to-microstrip transition, input and output filters, matching networks and an antiparallel diode pair. According to the simulation result of computer aided design software, the integrated circuit is fabricated using RF-Duroid 5880 with 0.254 mm thick and dielectric constant εr of 2.22. The conversion loss of 8.5 dB to 10.5 dB is achived with the IF fixed at 100 MHz, the RF ranged from 34.2 GHz to 35.2 GHz and the LO ranged from 8.525 GHz to 8.775 GHz in measurement.