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A series of three-dimensional molecular dynamics (MD) simulations was performed to investigate the effect of temperature on the nanoscale deformation behaviour and mechanical properties of c -plane monocrystalline gallium nitride (GaN) under nanoindentation. Simulation results showed that the atomic displacement and strain increased with increasing temperature. Thus, the nucleation and propagation of dislocations in the main slip systems were promoted to intensify the plastic deformation of GaN crystal. Increasing the temperature also contributed to the increase in local phase transition from the wurtzite crystal structure (B4-GaN) to the zinc-blende crystal structure (B3-GaN) and a very evident amorphization of GaN in the indentation zone. Based on the analysis of the load-depth curves, it was found that both the hardness and Young's modulus decreased as the temperature increased. This work can enrich the atomic-level understanding of the effect of temperature on the mechanical response of monocrystalline GaN when subjected to the external loads. [ABSTRACT FROM AUTHOR]