Abstract: Because it is difficult to detect the condition of Control Rod Drive Mechanism (CRDM) in reactors by normal detection method, this paper presents one new method to monitor the the condition of CRDM action component by on-line measuring of the coil inductance value. The corresponding relationship between the coil inductance value and the action component has been acquired by experiment, and the condition monitoring has been conducted under  the prototype simulation failure condition. The results show that the condition monitoring method can be used for the monitoring of the CRDM action component condition accurately, and the method can be easily implemented without additional equipment requirment in the reactor.