On the basis of failure data gathered from 5 NC machine tools of one type in one year，a two-parameter Weibull distribution model is established，and this model has passed linear correlation test and hypothesis testing. On the principle of the most effectiveness，the best preventive maintenance intervals model on the state of reparative maintenance is brought forward，and the best preventive maintenance intervals of this type have been calculated. In the end，an appropriate maintenance plan is brought forward according to the best preventive maintenance intervals，and the maintenance cost is greatly reduced and the availability of this type of NC machine tool are kept at the same time.