The mathematical model for rotating speed and hydraulic character of by-pass centrifugal oil filter was deduced.Through experiments,the rotating speed and hydraulic character and the filter efficiency of the FL115 centrifugal filter under different nozzle calibers and temperatures were obtained.The relationship among the rotor speed, the driving flow and the feeding pressure was analysed.The optimum caliber dimension in design was educed.Through comparison between the theoretical data obtained from the mathematical model and the experimental data,the mathematical model is proved to be serviceable in designing the centrifugal filters.