Abstract: The I-shaped non-standard small samples and the bar creep standard samples were sampled from the directional solidification nickel-based alloy bar in heat treatment state. The stress relaxation tests at different temperatures (800,850,900,980 ℃) and creep rupture tests under different stresses (250,350,450 MPa for standard samples and 250,350,400,450 MPa for small samples) were carried out. The relaxation creep and creep rupture behavior of standard and small samples were studied, and the relationship between the relaxation creep rate of small samples and the creep rupture life of standard samples was established. The results show that the creep rupture curves of both small and standard samples increased monotonically until samples fracturing, and the creep rupture rate curves were U-shaped curves. The minimum creep rupture rates of the two samples were close. The relaxation creep curves of both small and standard samples at high temperature decreased monotonically, and after temperature normalization, the relaxation creep rates of both distributed on one line. After creep rupture and stress relaxation tests, the γ′ phases in standard and small samples aggregated and coarsened along the direction perpendicular to the stress, which belonged to N-type raft. There was a linear relationship between the persistent life of standard sample and the relaxation creep rate of small sample, and the creep rupture life predicted by using the relaxation creep rate of small sample was close to the actual creep rupture life of standard sample.