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 the increasing hold time. The number/size of microvoid/cavity, the fraction of cavity area varied with the hold time. Therefore, the life evaluation using the microvoid with the variation of hold time is very important for safety of components in power plants. Especially, the HAZ(Heat Affected Zone) of weldment is reported to be the weakest part of the welded industrial components and serves as the easy microvoid initiating points. Therefore, the inspection of the defects such as micro-crack, micro-void/cavity is very important for safety of welded component and reliability of the long time operation of the high temperature components/facilities such as power plants and boilers. In this study, using the newly developed alloy of P92 and P122, both the artificial creep-fatigue degradation tests and its ultrasonic inspection were carried out for the purpose of evaluation for 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. It was found that as the hold time increased, the creep-fatigue life decreased and SDA decreased due to the degradation area with the effect of creep. 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 obtained. Under the same hold time condition, the fraction of cavity area of HAZ was greater than that of base metal and the creep-fatigue life of HAZ was shorter than that of base metal. The metallurgical inspection carried out to verify the degradation with the increasing hold time showed that as the hold time increased, the material degradation was accelerated by the increased formation of microvoids/cavities. 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.