We present an approach of second harmonic generation for edge localization of nano-scale defects measurement, based on the impact of the oscillating tip on the sample that induces higher harmonics of the excitation frequency. The harmonic signals of tip motion are measured by the heterodyne interferometry. The edge amplitude ratio for the edge characterization can be calculated by a mechanics model and the threshold of edge localization is experimentally determined by second harmonic profiles. This approach has been successfully utilized to measure the pitch of a standard sample. The results show that the second harmonic is sensitive to locating the edge of nano-scale defects with high accuracy.