The detection of interfacial micro-delamination in the thin-layered structure such as the electronic package becomes very important as the electronic device becomes smaller and thinner. The conventional method used to detect the delamination in an electronic package is to use a scanning acoustic microscope (SAM). However, despite its high performance qualities, SAM is often faced with a tricky decision when a delaminated gap is too small. In this paper, a novel method based on ultrasonic nonlinearity is proposed to overcome this limit. The proposed method is integrated into the conventional SAM equipment, and its effectiveness is verified by experiments for the Newton Ring and the real semiconductor package that have micro-delaminations. The results showed that the nonlinear parameter had good correlation with the gap size of delamination. A method of imaging the nonlinear parameter is also proposed to assure the feasibility of the proposed method in the field application.