This paper presents a bootstrapped inverter based ring oscillator for 0.2-0.6V operation. The proposed delay cell provides a boosted voltage swing to enhance the driving capability and suppress the large process variation in the low-voltage region. As compared with conventional inverter-based ring oscillators, the proposed one provides better linearity when controlled by the supply voltage. A test chip is fabricated in 90 nm SPRVT Low-K CMOS process. The measured results demonstrate that the proposed bootstrapped ring oscillator oscillates at 48MHz (771MHz) with a power consumption of at 0.63μW (87.6μw) and a phase noise of -93dBc/Hz (-88.5dBc/Hz) at a 1-MHz offset under a supply voltage of 0.2V (0.6V) V DD.