Creep phenomenon is a common mechanical behavior in nanocrystalline nickel (Ni) at high temperature that may affect component function and lead to material failure. In this paper, we systematically study the effects of temperature, stress, and grain size (GS) on creep behavior at high temperature in nanocrystalline Ni by molecular dynamics simulation. Three nanocrystalline Ni models with different GS and a single-crystal Ni model are built for creep behavior simulation. Stress exponent, GS exponent and microstructure characteristics during creep simulation are used to describe steady-state creep mechanism. An obvious creep phenomenon is observed in nanocrystalline Ni, but not in single-crystal Ni; primary creep and steady-state creep phenomenon both occur in nanocrystalline Ni during creep simulation. With increase of temperature and stress level, decrease of GS, creep mechanisms are discovered to change from (1) lattice diffusion and grain boundary (GB) sliding to (2) GB diffusion and GB sliding and then to (3) dislocation nucleation. The similar creep mechanism transition tendency is also found in smaller GS sample at lower temperature. The variation of stress exponent and GS exponent obtained from the simulation results is consistent with creep mechanism theory.