An automatic tool changer (ATC) is one of the most important subsystems of a computerized numerical control (CNC) lathe. The field failure analysis is very important to improve the reliability of the ATC. The structure and mechanism of an ATC is briefly introduced. Its brief reliability block diagram is set up. With this understanding, through the collection of the field failure data, a coding system to code failure data is devised and a failure analysis database of CNC lathes is accrued. The failure mode and cause are analyzed to indicate the weak subsystem of the ATC. Some suggestions on improving the reliability have been proposed.