This paper presents a high temperature voltage-controlled oscillator (VCO) for downhole communications. The proposed VCO adopts the common-source Colpitts oscillator configuration. The VCO is prototyped using 0.25 μm GaN RF transistors due to the high junction temperature and high unity gain frequency. Two GaN varactors are used to achieve the tuning frequency range of 40 MHz up to 230 °C. The measurements at 230 °C show that the VCO has an output power of 17.5 dBm with ±0.5 dB variations over the tuning range from 320.8 to 360.2 MHz. The measured phase noise is -121 dBc/Hz at 100 kHz offset from 343.3 MHz carrier. The maximum power consumption of the VCO is 122.5 mW.