This paper presents two low-voltage and low-power CMOS voltage controlled ring oscillators (VCOs) for use in portable wireless sensor applications. The frequency is controlled using an inversion mode MOS voltage controlled capacitor (varactor) to generate a large tuning range between 166 MHz to 600 MHz for the first design and from 250 MHz to 850 MHz for the second one and consume only 151.2 μW and 146.19 μW respectively. The proposed three-stage VCOs are designed and simulated in TSMC 65nm technology. Both operate on a 0.6 V power supply.