In this paper, an ultra low power 15-bit digitally controlled oscillator (DCO) is proposed. The proposed DCO is designed based on a segmental coarse-tuning stage which employs novel Schmitt-trigger based hysteresis delay cells (HDC) as well as digitally controlled varactor (DCV) in the fine-tuning stage. Simulation of the proposed DCO using TSMC 180nm model achieves controllable frequency range of 191MHz ∼ 850MHz with a wide linearity. Monte Carlo simulation demonstrates that the time-period jitter due to random power supply fluctuation is under 124.8ps and the power consumption is 137µW at 215MHz with 1.8V power supply.