Abstract: Baffle bolts are the critical joint parts of the internal structures in nuclear power plants, and it is necessary to perform non-destructive testing on their structural integrity in order to detect the defects such as irradiation assisted stress corrosion cracking (IASCC) which may occur during a long-term service. In this study, the structural characteristics of the baffle bolts and the in-service inspection conditions were analyzed, and a combined-crystal ultrasonic technique (UT) was developed for the external hexagon bolts. The probe design and defect assessment were reviewed to ensure the good sound field coverage, and the cracks in different parts of the bolt can be detected. The practical trials on the flawed samples were performed to verify the feasibility of the ultrasonic process, and the result showed that currently technique is able to detect the flaw on the order of 30% of the shank cross-sectional area and the signal to noise ratio (SNR) can reach more than 12dB, which meet the requirements for the in-service inspection.