Abstract: The cause of a reactor trip event occurred in a nuclear power plant due to loss of power supply to the control rod drive mechanism (CRDM) was systematically analyzed. The root cause was investigated and analyzed by using the event and cause factor (ECF) chart analysis method. The sequence of event was established to determine the failure points of the event process. The contributing causes and root causes of the event were determined by deep analysis of each failure point. It is found that the root causes of the event are that the quarantine manager is lack of understanding of risk caused by pressing 3RAM601JA and by implementing action out of program specification, no self-checking methods or other protections from human errors are taken during the operation and the manager is in shortage of job skills. On the other hand, there is a nuclear safety culture deficiency at the power plant, showing that the relevant operator took power recovery as a priority, and actively violated the SOP program.