Abstract: 940 WANO operating event reports from 1993 to 2002 were analyzed, among which 551 were found relative to human errors. Human errors are still one of the major causes of accidents in a nuclear power plant （NPP）. In the paper, the following conclusions are concluded: （1） human errors in test, maintenance and calibration leading to the potential failure of the system are the main causes of human factor incidents; （2） there is a comparatively high probability of human errors in the period of start-up and shutdown of a reactor which are liable to cause severe accidents; （3） the probability of human errors is not necessarily related to the type of a reactor ;（4） the main human errors include the error judgment on the symptoms and manipulation which most probably cause severe transient and the failure of safety system or improper shutdown; （5） lack of theoretical knowledge, poor manipulation, organization management failure, inappropriate procedures, carelessness and lack of check-up are the major root causes of human errors.