Decreasing reserves of natural resources drives the oil and gas industry to drill deeper and deeper to reach unexploited wells. Coupled with the demand for substantial real-time data transmission, the need for high speed electronics able to operating in harsher ambient environment is quickly on the rise. This paper presents a high temperature VCO for downhole communication system. The proposed VCO is designed and prototyped using 0.25 μm GaN on SiC RF transistor which has extremely high junction temperature capability. Measurements show that the proposed VCO can operate reliably under ambient temperature from 25 °C up to 230 °C and is tunable from 328 MHz to 353 Mhz. The measured output power is 18 dBm with ±1 dB variations over entire covered temperature and frequency range. Measured phase noise at 230 °C is from -121 dBc/Hz to -109 dBc/Hz at 100 KHz offset.