A system for on-line measurement and sorting of ball bearings is presented in this paper. The structures and functions of its automatic testing mechanics and control system are described in detail. The system sorts the tested bearings into four grades by measured values of vibration parameters such as root-mean-square value, peak value, crest factor and kurtosis. Its stability of indication values is between ± 1.0 dB. The mechanism generating abnormal sounds of bearings is also investigated emphatically and the characteristics of abnormal sound signals are described on the basis of vibration model of tested bearing.