In order to realize load adaptive ultrasonic grinding, an intelligent adaptive control system was established based on resonant frequency.Its control principle, control algorithm and control effect were investigated. The equivalent circuit model of ultrasonic grinding system was established based on mechanicelectric analogous theory and the resonant frequency equation of ultrasonic grinding system was deduced. Then, the relationship between the load and the resonant frequency was studied by theoretical calculation and experimental analysis. Based on the influence of load on the resonant frequency, a fuzzy controller for ultrasonic grinding was developed with resonant frequency feedback and it was applied in Siemens CNC system for test. The experimental results indicate that the control error of resonant frequency is 12% and the fluctuation range of grinding force is controlled within 5 N. The control system is stable and robust, and can effectively improve the grinding quality and efficiency, and reduce the wear of grinding tool.