It was attempted to evaluate nondestructively the creep damaged IN738LC Ni based superalloys with different degree of rafting using ultrasonic wave. Microstructural parameters (length and width of gamma' precipitates) and ultrasonic attenuation were measured in order to clarify the relationship between them. Both the mean length of gamma' precipitates and the ultrasonic attenuation increased with creep time. Ultrasonic attenuation was found to have a linear correlation with the mean length of gamma' precipitates. It is suggested that the ultrasonic attenuation can be used as a potential nondestructive parameter for assessing the degree of creep damage of IN738LC superalloy.