A high precision RC oscillator with self-calibration based on frequency offset was proposed. The count offset between RC oscillator and reference clock was obtained by counting the PTAT high frequency ring oscillator. Further, the oscillator was stable by introducing a digital self-calibration circuit with the reduce of the count offset, which was derived from calibrating the reference voltage through a resistance array. The reference clock was only used on time during calibration and was not required in practical work. The proposed RC oscillator was fabricated in 0.18-μm CMOS process with a supply voltage of 1.8 V. The simulation results show that the circuit can generate a stable frequency of 2 MHz, and the power consumption of the entire system is 48.4 μW. The stable time is less than 15 μs. The frequency variation is less than ±0.2% in the temperature range of -40 ℃-125 ℃. The frequency variation is less than ±0.25%/V within the power supply voltage range of 1.70-1.98 V.