Abstract: Aiming at the non-destructive testing problems of interfacial defects of diffusion bonding, the water immersion ultrasonic testing was performed on the prepared specimen with artificial defects. An ultrasonic response model of interfacial defects of diffusion bonding was proposed. A measurement method was developed to evaluate the interfacial defect thickness by ultrasonic testing. The specimen with artificial defects was firstly prepared by the diffusion bonding process. It was later subjected to the water immersion ultrasonic testing through an incident wave frequency of 30 MHz. Based on the principle of ultrasonic non-destructive testing, an ultrasonic response model for unbonded defects was proposed. The interfacial stiffness coefficient (K) was introduced as a bridge to establish the relationship between the ultrasonic reflectivity and the interfacial defect size. At the specific measurement position, constants of the ultrasonic response model were determined by fitting the ultrasonic reflected wave data and actual micro defect size. Therefore, the interfacial defect thickness can be evaluated by the proposed model based on ultrasonic non-destructive testing. The results show that tradition ultrasonic C scanning usually determines qualitatively whether the defects exist or not. The proposed measurement method of interfacial defect thickness make a supplement for ultrasonic C scanning to some extent. It is helpful for measuring defect size quantitatively. The evaluation of defect risk level is achieved.