The concept and method of diagnosing the regenerative chatter are discussed in this paper. Both theoretical analysis and experimental results demonstrate that a certain corresponding relationship between the system stability and the phase shift angle ψ in the two wavy surface: one from the previous cut and another from the current cut, and hence, one can identify whether the chatter occurring in the cutting process is a regenerative chatter. The phase shift angle ψ is concerned with the chatter frequency and the rotational speed of the workpiece. In order to improve diagnostic accuracy, digital ZOOM FFT has been applied