Abstract: Signal processing was adopted in order to study the nonlinear behavior of ultrasound in diffusion bonding. First of all,diffusion bonding samples which contain typical defects were made by controlling the surface treatment before welding and changing the welding process. Ultrasonic testing was performed to obtain the data of ultrasonic signal which correlate with the joint states. Ultrasonic nonlinear effects of different states in diffusion bonding of stainless steel were analyzed from the points of view of the theory of ultrasonic nonlinearity in the joints of solidphase connection,ultrasonic nonlinear images,microstructures and shear strength. The results showed that nonlinear ultrasonic testing was more sensitive to kissing bonds. It is possible to achieve nondestructive testing in diffusion bonding based both on C-scan images and ultrasonic nonlinear images to some extent.