A dual-gate subharmonic injection-locked oscillator (SILO) designed for Q-band millimeter-wave applications has been implemented in 0.5 μm GaAs pHEMT process. Based on the dual-gate circuit topology, a wide-bandwidth and high-frequency negative resistance can be easily obtained for determining the free-running oscillation frequency by a proper resonator. The free-running frequency is designed around 49 GHz. The power consumption is 75 mW from a 5 V supply. The measured frequency tuning range is from 48.7 GHz to 49.7 GHz with 8 dBm output power. By injecting a 2nd-order subharmonic signal into the oscillator without any frequency tuning, the maximum locking range can reach to be 2.8 GHz. The measured output phase noise under locking status is close to 121 dBc/Hz at 1-MHz offset frequency.