Abstract: The effects of lattice mismatch on the nonlinear ultrasonic response of nickel-based superalloy GH706 (Inconel 706) were examined. The nickel-based superalloy was heat treated under solution treatment, stabilizing treatment, and aging treatment conditions. Mean grain size, shape, and hardness were measured. Lattice mismatch of coherent interface were studied with transmission electron microscopy (TEM) images. The ultrasonic and nonlinear ultrasonic responses were detected and used for calculations. The results showed that there is a strong correlation between nonlinear coefficient and hardness when aging strengthening is the main influencing factor of alloy hardness. The nonlinear coefficient is sensitive to the change of grain boundary. Lattice mismatch during aging treatment is the main nonlinear source of ultrasonic. The nonlinear coefficient increases with the increase of lattice mismatch, reaches a peak at the occurrence of nonconforming interface, and then decreases gradually. The nonlinear ultrasonic technology can be used for non-destructive qualitative evaluation of lattice mismatch in GH706.