Maintenance intervals and the optimal number of maintenance N*of NC machine tools in infinite time horizon are presented using a hybrid sequential imperfect preventive maintenance (PM) model. The failures of NC machine tools are described using Weibull process. Sensitivity analysis of cost parameters is given to show how the optimal results depend on the different cost parameters. The results show that when the shape parameter β of Weibull process within the range of [1.1,1.3], N* is slightly affected by β and cost ratio cr /cp. However, when the value of β is greater than 1.4, the effect of β and cr /cp to N* is significance. If an equipment is in wear later stage or PM actions are not too effective to it, it is not suitable to add the number of maintenance.