Generally, as the hold time of static load increases, the degradation of material becomes more rapid and the creep-fatigue life decreases. Under the creep-fatigue interaction, the cause of life decrease was mainly the initiation and growth of microvoid with increasing hold time. In this study, using the newly developed alloy of P92, the artificial creep-fatigue degradation tests and its ultrasonic inspection were carried out to evaluate creep-fatigue life. From the relations between the creep-fatigue life and the nondestructive degradation assessment by backscattered ultrasound inspection, the new nondestructive life evaluation parameter, SDA (Slope of Decaying Amplitude) was proposed. Also, to verify the nondestructive life evaluation ability, the life evaluation results by SDA were compared with those of the destructive life evaluation obtained through the fraction of cavity area. From the test result of backscattering Rayleigh surface wave, the linear relationship between SDA and experimental life could be obtained. The good agreement between the experimental life and the predicted life by SDA was also obtained. Finally, it can be stated that the new life evaluation/degradation parameter, SDA (Slope of Decaying Amplitude) could be utilized for the evaluation of the material degradation under creep-fatigue interaction.