Abstract: Diagnostic principle and method in regenerative chatter are discussed emphatically in this article. It is confirmed that stability of regenerative chatter is concerned with the phase difference ψ between two wavy surfaces: one from the previous cut, and another from the current cut. Measuring the phase difference ψ during machining, we can know whether the chatter excited belongs to regenerative chatter. The phase difference depends on the chatter frequency and revolutions of workpiece per minute. In order to improve diagnostic accuracy, Digital ZOOM FFT had been applied.