This paper addresses the performance of a fully integrated low phase noise X-band oscillator fabricated by using an InGaP/GaAs HBT process with f(t), of 53.2 GHz. The oscillator circuit consists of a negative resistance generating circuit with base inductors, a resonating emitter circuit with micro-strip lines and a buffing resistive collector circuit with tuning diodes. The oscillator achieves 4.33 dBm output power and exhibits -121.17 dBc/Hz phase noise at 100 KHz away from 10.38 GHz oscillating frequency. This phase noise is, to our knowledge, the lowest reported for monolithic oscillators with oscillation frequencies higher than 10 GHz. The oscillator draws 36 mA current from a 6.19 V supply and occupies 0.8 min by 0.8 mm die area. (C) 2004 Published by Elsevier B.V.