Abstract: Three vibration diagnosis methods are discussed, which are normal spectrum analysis method, modal parameter recognition method and time series analysis method. It is demonstrated that the modal parameter recognition method is far from a practical one due to its threoretical shortcomings. The time series analysis method is promising to diagnose the machine defaults while ased as a tool to recognize the "input form filter". Therefore, this method is comparable with the maximum entropy spectrum analysis but performed in the time domain. A perfect diagnosis software should contain both the normal spectrum and time series analysis methods.