In this study, ultrasonic nonlinear characteristics in the heat-treated aluminum alloy have been evaluated. The nonlinearity of ultrasonic wave has been measured as the acoustic nonlinear parameter

, depending upon the amplitude ratio of the second-order harmonic and the fundamental frequency component of ultrasonic wave propagating through the materials. The parameter

measurement has been carried out with the reflected signals from the back-wall of specimens at the same plane using the contact-type transducers. The heat-treatment, aging, has been achieved at

for various durations in the range of 1 to 50 hours. The tensile strength and elongation are obtained by the tensile test and then compared with the parameter

. There is a peak of the acoustic nonlinear parameter

on 5 hours aging and the

decreases thereafter, exhibiting closed relations with tensile strength and elongation. Also, the heat-treatment time showing peak in the parameter

was identical to that showing severe change in the

curve. These results suggest that the acoustic nonlinear parameter

can be used for monitoring the strength variations with aging of aluminum alloys.