The means of classification of the NC machine tool malfunctions is the key basis on detecting malfunctions and processing malfunctions` s signal. By defining malfunction mechanism and malfunction mode, a series of new classifications are presented. According to malfunction mechanism, mechanical malfunction are damage malfunction, lubrication malfunction, and fit malfunction. According to malfunction mode, the noise malfunction can be classified as vibration malfunction mode, the heating malfunction can be classified as heating malfunction mode, and the bad motion malfunction can be classified as deformation malfunction mode. Conclusion is that damage malfunction of gear and bearing malfunction is the most frequent malfunction whose performance formation is noise. Another conclusion is that tooth surface fatigue is the familiar malfunction, and bearing malfunction is mainly caused by surface peeling. Based on these conclusions, the vibration model of the mechanical failure is established, which lays a foundation for dealing with the follow-up signal.