Sensitivity of nonlinear ultrasonic method towards isothermal annealing in pure copper is investigated in this study. It was reported that, recovery, recrystallization and grain growth associated with annealing has profound influence on nonlinearity parameter. Presence of precipitates and secondary phase particles, and its effect on nonlinearity parameter was considered by many authors. However, in this study, focus is on the influence of grain-growth on nonlinearity parameter. Variation of non-linearity parameter, β, with different holding durations were analyzed. All the samples were annealed at a temperature above its recrystallization temperature. Further, metallography studies as well as hardness measurements were performed to understand the relationship between microstructural changes and ultrasonic nonlinearity parameter. © 2014 AIP Publishing LLC.