A Q-band low phase noise voltage controlled oscillator (VCO) using balanced π-feedback with 2-μm GaAs heterojunction bipolar transistor (HBT) process is reported in this paper. The VCO features a phase noise of -105.5 dBc/Hz at 1-MHz offset, and a tuning frequency of from 41.2 to 42.1 GHz with a maximum output power of -9 dBm. The differential outputs are also provided from the VCO due to the use of balanced π-feedback. The overall dc power consumption of the VCO is only 20 mW with a supply voltage of 2.5 V. The chip size of the VCO is 1 × 1 mm2. To the best of the authors' knowledge, this work demonstrates the lowest FOM among au the reported VCOs except the InP-based HBT VCOs around 40 GHz.