A reliability evaluation method based on mean time to first failure (MTTFF) was proposed for the CNC system reliability evaluation problem. The health status of 20 CNC machines which were equipped with FANUC 0i CNC system were monitored for 24 months, and the experimental data about first failure time were acquired. Then the distribution function parameters of the first failure time data were estimated. The interval of the distribution function was estimated based on Monte Carlo method, and the interval estimation value of the first failure time about the CNC system with high degree of confidence was gotten. This estimation value is of guiding significance to the reliability design and evaluation of CNC systems.