The harmonic generation technique has been used to characterize the isothermal degradation of pressure vessel steel. The isothermal degradation was conducted at 630 C with forged 2.25Cr-1Mo steel. The variation in the normalized ultrasonic nonlinearity parameter (= 0 ) was interpreted as having resulted from microstructural evolution, which was supported by the results of electron microscopy and X-ray diffraction, in addition to Vickers hardness and ductile-brittle transition temperature. The normalized ultrasonic nonlinearity parameter increased progressively with the isothermal degradation time due to the increase in the volume fraction of equilibrium M 6 C carbide and the variation in the lattice parameter of M 23 C 6 carbide. It was found that the nonlinearity parameter was very sensitive to the microstructure during the isothermal degradation of 2.25Cr-1Mo steel. The harmonic generation technique has the potential to assess microstructural changes due to isothermal degradation.