The nanoindentation process of single-crystal and nanocrystalline copper is studied with molecular dynamics simulations based on the many-body tight-binding potential. The grain size effect is evaluated in terms of slip vector, atomic stress, loading force, and hardness. The inverse Hall-Petch relation is found below 7 nm. With grain size smaller than 5 nm, the equivalent stress decreases quickly and stress-induced grain growth is observed in indentation process. Grain rotation is the main cause of grain coarsening for small grain sizes. For larger grain sizes, dislocations are found at the {1 1 1} close-packed plane and {1 0 0} plane. (C) 2015 Elsevier B.V. All rights reserved.