An operating method for a fault detection of a semiconductor process and a diagnostic system for fault detection in a semiconductor process are described. By using the method and the diagnostic system, the real-time process parameters collected during the process is performed by the tool become meaningful and are correlated with the historic process performance data obtained by the post process metrology process. Moreover, the method and the diagnostic system further provide an alarm index for the process performed on the tool to actually reflect the process environment during the process is performed after correlating the real-time process parameters and the historic process performance data. With referring to the alarm index, the current process performance under the real-time process parameters in the tool can be accurately diagnosed.