The nonlinear acoustic effect is a sensitive tool to detect a micro-scale crack or the early stage of cracking during the fatigue process. The damage produces a nonlinear stress-strain relationship and the nonlinearity can be measured by a higher harmonic component in the frequency domain. The more damage, the larger is the level of a nonlinearity, and it can be used to for image or diagnosis of micro-cracking. Because the laser beam can be focused on the smallest spot, the localized nonlinear acoustic parameters can be determined. By scanning the laser beam, localized nonlinear acoustic parameters can be mapped around a crack in a CT (compact tension) specimen. Various nonlinear parameters are chosen and compared for the most sensitive nonlinear parameter for the image of crack tip or micro-cracking.