A monotonic and low-power digitally controlled oscillator (DCO) with a cell-based design for system-on-chip applications is presented. The proposed monotonic DCO not only can achieve high delay resolution and wide operation range at the same time, but also maintain the delay monotonicity. Moreover, based on the proposed varactor-interpolator fine-tuning stage, the proposed DCO can provide monotonic delay with low power consumption and low circuit complexity as compared with conventional approaches. Simulation results of the DCO show that power consumption can be improved to 0.21 mW (at 840 MHz) with 2.95 ps resolution. In addition, the proposed design can be implemented with standard cells, making it easily portable to different processes and very suitable for SoC applications.