A low-voltage phase-locked-loop (PLL) circuit with a supply-noise-compensated feedforward ring voltage-controlled oscillator (FRVCO) is demonstrated. The oscillation frequency fluctuation due to supply noise is compensated by adjusting the ratio of driving strength in feedforward and direct paths in FRVCO. A prototype 400-MHz PLL circuit operating at 0.65 V is fabricated with 180-nm standard CMOS process. Measurement results show that supply-noise compensation is successfully achieved. Our PLL consumes only 242.1 μW.