We present a 125 GHz voltage-controlled oscillator (VCO) with a 3.4% tuning range based on a 65 nm CMOS process. The oscillator is tuned by utilizing the capacitance variation in NMOSFETs which is controlled by using the p-type bulk voltage of the NMOSFETs. The VCO has a simple structure to achieve a small internal loss. The VCO has an oscillation frequency of 124.9 GHz, an output power of -1.6 dBm and a power consumption of 17.0 mW at a drain voltage of 1.0 V and a bulk voltage of 0 V. The tuning range is from 122.0 GHz to 126.3 GHz.