A W-band planar injection-locked harmonic oscillator (ILHO) based on substrate integrated waveguide (SIW) was implemented. Using the technique of harmonic extraction from diodes as a frequency multiplier, this ILHO has a free-running output frequency around 90.2 GHz under the condition of strong coupling for high injection power. When injecting a signal near 45.08 GHz in the fundamental injection-locked mode, it has an output power more than 6.5 dBm with a locking bandwidth of more than 120 MHz. The combination of the simple synchronization with a low-frequency reference signal allows the generation of stable and low phase-noise W-band signals with a fully integrated planar source.