An on-chip high accuracy fully integrated relaxation oscillator was designed based on average-voltage-feedback technology, which overcame the traditional relaxation oscillator's sensitivity to comparator delay, device aging, and flicker noise. In addition, a one-time automatic frequency calibration circuit was designed, which could make the oscillator achieve high accuracy frequency with the help of an external reference clock. Based on UMC 40 nm CMOS process, a 50 MHz high accuracy fully integrated relaxation oscillator was implemented, and the layout and post simulation results of the oscillator were also provided. The core area of the oscillator was 181 μm×218 μm. The post-simulation results show that the presented oscillator can calibrate automatically the output frequency to 50 MHz under different process corners, and the output frequency error is only ±0.47% when the supply voltage changes from 2.2 V to 3.6 V and the temperature changes from -40 ℃ to 125 ℃. Under a typical corner, the power consumption of the oscillator is only 200 μW.