Abstract: Based a pair of selfinvented clamps, a series of strain fatigue tests at 400 ℃ based on thinwalled tubular specimens of N18 alloy were carried out. The results show that cyclic strainstress curves are symmetrical due to the clamps, and the alloy behaves cyclic hardening at lower strain amplitude and behaves cyclic softening at higher strain amplitude under cyclic straining, therefore N18 alloy does not follow Manson law, however, under multigrade strain cycling (MSC) with different loading ways, stress amplitude response of the alloy remains stable or cyclic constitutive relationships of the alloy tubes are closed each other. Finally, several lifeestimated equations for the alloy tubes were presented.