The effects of grain size and grain boundary stability on the mechanical and fatigue properties of nanocrystalline nickel thin films were examined. We found that the grain size of nanocrystalline nickel thin films tended to decrease with the amount of sodium allylsulfonate, and the grain size on the solution side was larger than that on the substrate side. The dependence of the grain size on the 0.2% proof strength, the tensile strength, and the fatigue limit could be expressed by the Hall–Petch equation. The 0.2% proof strength and tensile strength of the annealed film are lower than those of the as-deposited film, while the fatigue limit of the annealed film was higher than that of the as-deposited film. The grain boundary stability and dislocation density are responsible for the effect of annealing. The fracture strain of the annealed film is lower than that of the as-deposited film, indicating that annealing makes the film brittle.