The method of optimized sensitivity modulation for the resonator optic gyroscope (ROG) is introduced. ROG is a novel optical rotation sensor whose resonant frequency is changed due to the Sagnac effect. The limited sensitivity of ROG should be obtained while the modulating frequency is optimized according to frequency-modulation spectroscopy. This adjusting method which is based on the theory of modulation frequency simplifies the process of optimized sensitivity modulation in ROG system. The theory and the structure of ROG system are analyzed. The modulation method of the optimized sensitivity based on modulation frequency is presented. The actual experiment results have proved that the method can modulate ROG to the optimized sensitivity and can apply in the adjustment of ROG system.