A high precision and externally synchronous relaxation oscillator with simple struc- ture is proposed in this paper. The oscillator consists of the current circuits, a capacitor, two comparators, a RS flip-flop and other components. In the current circuits positive and negative temperature coefficient currents are obtained. With the current temperature compensation, a low temperature current is used to charge and discharge the capacity. By optimizing the delay of the comparators the oscillator has a good ability of control linearity In this method a high precision oscillator is obtained. Based on 0.5 µm OKI technology library, HSPICE and Cadence software is used to circuit simulation the simulation results show that the oscillator has stable output frequency and external synchronization. Under the environment of typical applications in the chip system, the frequency of the oscillation is 500.1 kHz, With the power supply temperature changing from −40◦ C to 125◦ C, the frequency migration ranges from −0.3% to +1.3%.