Abstract: The detection of defect on the diffusion bond interface is one of the difficulties in nondestructive testing. In order to investigate the detectability of ultrasonic C-scan on titanium alloy diffusion bonding joint, by taking titanium alloy as the research object, diffusion bond samples with different size and type defects were manufactured, and the influence of scan step and detection frequency on the detectability was studied, then it was verified with destructive testing results. The results demonstrated that the detection accuracy can be improved by using smaller step size and higher detection frequency. However, there is a great deal of contact-type and micro defects on the bonding interface, thus it is difficult to precisely quantify it with common ultrasonic testing.