As a standard topology in low frequency region, the double-balanced Gilbert mixer is seldom applied in millimeter-wave and terahertz range. For compound semiconductor Monolithic Microwave Integrated Circuit(MMIC) design, few literature focuses on Gilbert mixer above 100 GHz. In this paper, a 120 GHz double-balanced Gilbert mixer is designed based on 70 nm GaAs High Electron Mobility Transistors(mHEMT) technology. And the layout is modified to improve the IF output balance performance. The simulated results show that within 100-135 GHz，the conversion loss is (-7.6±1.5) dB with PLO=0 dBm. The P1dB of RF input is 0 dBm@120 GHz. The bandwidth of IF is above 10 GHz and the differential IF output ports own an imbalance performance better than 1 dB and 4°. The chip consumes a DC power of 90 mW and the chip size is 1.5 mm×1.5 mm.