Non-linear interaction of waves with contact interfaces has been widely applied in nondestructive evaluation fields such as bonding quality evaluation, and the detection of closed microcracks and composite delamination. This paper proposes an absolute measurement of the ultrasonic non-linearity parameter using a piezoelectric detection method for two aluminum alloy blocks of different lengths. The results of a twodimensional finite element method model verified by models for hard and soft contact interfaces, depending on the interface property, were compared with the measured nonlinearity parameter. The measured values show good agreement with the modelled results, indicating good potential for measuring the non-linearity parameter at interfaces experimentally and numerically.