An attempt was made to assess the surface damage in the isothermally degraded FSX414 Co-based superalloy using ultrasonic Rayleigh surface wave. Microstructural analysis (amount of solute Cr near the surface) and measurement of ultrasonic velocity and attenuation were performed after isothermal degradation heat treatment at 1100°C for up to 4000 hours. Rayleigh wave velocity was observed to increase with aging time, which was attributed to the increased depletion of Cr near the surface. Strong frequency dependence of Rayleigh wave velocity was found in the specimens with surface depletion layer. Attenuation coefficient of Rayleigh wave increased with increasing aging time. Therefore, both attenuation and velocity of ultrasonic Rayleigh wave are suggested as potential nondestructive evaluation parameters for the characterization of the thermally aged FSX414 superalloy.