Abstract: Resonator fiber optic gyro (R -FOG) is a novel optical sensor whose resonant frequency changes due to the Sagnac effect. The closed loop frequency locking system can decrease the effect of temperature stress and other external environmental changes on fiber ring resonator and so improve the performance of the gyroscope. The design of closed loop frequency locking system where digital circuits and analog circuits were combined was proposed. It overcame the temperature drift in analog circuits, reduced A/D converter converting speed of system, also did not need complex digital demodulation algorithm. Using this design, locking frequency accuracy of 2.15 ()/s can be obtained in 10 s.