This paper reports a study for the evaluation of ultrasonic nonlinear characteristics in the heat-treated aluminum alloy. The nonlinearity of ultrasonic wave was estimated by using the acoustic nonlinear parameter β defined as the ratio of the second harmonic magnitude to the power of fundamental frequency component after propagation of ultrasonic wave through the material. A measurement system with contact transducers was constructed, which has a capability of reliable measurement by keeping measurement condition in consistent and reducing extra harmonics generated in the measurement system. The experimental results showed that the parameter β was affected by the heat-treated temperature (250°C, 300°C, 350°C) and the heated time up to 200 hours, of which pattern was very similar to the age hardening curve.