Abstract: It was necessary to detect the defects of Rod Cluster Control Assembly used in nuclear power plant reactor for its normal operation. The immersion pulse-echo technique was applied, from which the resonance frequency of the UT signal was obtained by performing FFT. After that, the defects information within the area of inspection can be analyzed from the UT signal. The method was validated on a calibration block whose thickness was known. The accuracy was high and the uncertainty was within the allowable range. With the help of this method, the ultrasonic inspection for Rod Cluster Control Assembly can be realized well.