The effect of W and B addition on the superplastic deformation and grain growth of electrodeposited nanocrystalline Ni alloys with a crystal grain size of approximately 20 nm was investigated with the aim of improving the mechanical properties by maintaining fine grain sizes during superplastic deformation. A maximum elongation of 12% was recorded for electrodeposited Ni1.8 at% W at a temperature of 350°C and a strain rate of 1.0 © 10 ¹4 s ¹1 . The electrodeposited NiW failed to exhibit superplasticity because the segregated W at the grain boundaries increased the energy required for grain boundary sliding. In contrast, the electrodeposited Ni0.06 at% B exhibited superplasticity with a recorded elongation of 362% at a temperature of 450°C and a strain rate of 1.0 © 10 ¹4 s ¹1 . With the addition of B, the optimal superplastic strain rates of the electrodeposited NiB shifted to lower values than that of the electrodeposited Ni. The grain size and hardness of the electrodeposited NiB after superplastic deformation were smaller and higher, respectively, than those of the electrodeposited Ni. The addition of B successfully suppressed grain growth and improved the mechanical properties after superplastic deformation.