Since nickel-based superalloys are more and more used as fasteners, it is necessary to investigate the stress relaxation behavior and mechanism of nickel-based superalloy. In present work, the stress relaxation mechanism for four typical nickel-based superalloys (GH4169, GH4169D, GH4738, GH350) for fasteners under service condition was investigated. The stress relaxation tests were carried out according to GB/T 10120-2013 in the temperature range of 600~780 ℃ and initial stress range of 260~510 MPa, and the stress relaxation curves were recorded. The microstructure was studied by FESEM and TEM. The results show that the stress decreases fast in the initial stage of stress relaxation test and then trends to be steady. The stress relaxation stability decreases with increasing temperature. There is no apparent change in the microstructure after stress relaxation test. TEM observation shows that the major mechanism of stress relaxation is the movement of dislocations, and the stress relaxation properties of different alloys depend on the inhibition of dislocation movement. The species, size, shape and distribution of phases determine the ability to hinder dislocation movement and the stress relaxation property of different alloys. GH4169 alloy gets the stress relaxation property mainly by γ' phase, γ'' phase and δ phase hindering the movement of dislocations. In GH4169D alloy, both γ' phase and η phase participate in the stress relaxation process. γ' phase in GH4738 alloy can effectively impede the movement of dislocations and provide good stress relaxation property. The combined effect of γ' phase and η phase guarantees the stress relaxation stability of GH350 alloy.