The linear ultrasonic technique has been extensively used as a powerful, non-destructive test tool for reliability testing and failure analysis of electronic packaging. This is used most often in the inspection of defects such as delaminations, voids, or cracks through use of a SAM (Scanning Acoustic Microscope). Then, as the reliability level that is required of electronic packaging becomes higher and the thickness of package becomes thinner, the possible defect which needs to be detected becomes smaller. In the conventional SAM, however it is very difficult to detect small defects less than m µ 1 . 0 , such as micro-delaminations. In order to solve such a problem, this paper proposes a nonlinear ultrasonic method, where the nonlinearity caused by the effect of crack-face interactions is considered. The basic concept of this method involves harmonic frequencies that are generated in the transmitted ultrasonic wave due to the partial contact at the interface of micro-delamination. As an evaluation index, the nonlinear parameter dependent on the amplitude of the second order harmonic frequency component is obtained by spectral analysis of the transmitted signal. Experimental results show that the nonlinear parameter has good correlation with the micro-gap and the proposed method can detect the micro-delamination even less than nm 1 .