This research investigates second harmonic generation in Rayleigh surface waves propagating in 9%Cr ferritic martensitic steel. Previous experimental results show that the nonlinearity parameter is sensitive to certain changes in a material's properties such as thermal embrittlement and hardness changes. Therefore, the nonlinearity parameter can be used as an indicator of thermal damage due to changes in dislocation density and precipitations. The specimens are isothermally aged for different holding times to create progressive changes in the microstructure and obtain different levels of thermal aging damage. As aging progresses the dislocation density decreases and precipitations are formed; these microstructural evolutions lead to changes in the nonlinearity parameter β. Nonlinear ultrasonic experiments are conducted for each specimen using a wedge transducer for generation and an air-coupled transducer for detection of Rayleigh surface waves. The amplitudes of the first and second order harmonics are measured at different propagation distances, and these amplitudes are used to obtain the relative nonlinearity parameter for each specimen at different aging stages. Conclusions about microstructural changes are drawn based on the nonlinear Rayleigh surface wave measurement and complementary measurements including scanning electron microscopy (SEM) and Rockwell HRC hardness. The results indicate that the nonlinearity parameter is very sensitive to the dislocation density and precipitate formation, and thus can be used to track the microstructural change in this material during the process of thermal aging.