Abstract: The nonlinearity acoustic parameter, which describes the micro-nano scale damage with a single scalar coefficient, is often measured by contact piezoelectric ultrasonic technique. However, the nonlinear feature from the couplant is unknown. Aiming at the problem of measurement process, a non-contact electromagnetic ultrasonic technique using longitudinal wave was proposed. The ultrasonic is stimulated in the specimen surface based oil Lorentz mechanism. The specimens were loaded by different levels of stretching stress and the relative acoustic nonlinearity parameter (ANP) were measured by piezoelectric ultrasonic and electromagnetic ultrasonic method respectively. The experimental results show that the relative ANP increases monotonically with the tensile strain of the aluminium alloy. This research also demonstrates that the nonlinear electromagnetic ultrasonic technique using longitudinal wave can be used to evaluate the plastic deformation.