This study used the dynamometer to capture force signals in the ultrasonic machining, and analyzed the cavitation, impact, and hammer effects by the empirical mode decomposition (EMD). By capturing the features such as marginal spectrum, power, and mean frequency of the vibration modes, a mechanism which can determine the ultrasonic machining stability and workpiece quality by its force signals was developed. This mechanism could be used to monitor the ultrasonic machining process.