This paper presents a method for determining the degree of nonlinearity (DON) from the structural vibration data for monitoring the change in health of structures. Using Hilbert Transform of the frequency response function, the DON measures the nonlinearity present in the vibration response. It is shown that a plot of the DON against the magnitude of the motion represents the behavior of a structure. If the structure is damaged during a new striking motion, the DON parameter will deviate from its healthy signature. Data from numerical simulation and experimental measurement were used to evaluate the proposed method.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.