Abstract: The sintering process of gold nanoparticles was investigated using molecular dynamics simulation, and the detailed crystal structure transformation and neck growth mechanisms were identified and analyzed for different particle sizes. It is found that the coalescence of two nanoparticles experiences two stages, which are the initial rapid neck formation and stable neck growth. Simulation results show that different particle sizes lead to different neck growth mechanisms. When the particle size is 4 nm, the major neck growth mechanisms are grain-boundary/dislocation slid, surface diffusion, and viscous flow. For the particles with the size of about 6 nm, the major neck growth mechanisms are grain-boundary diffusion, surface diffusion, and viscous flow. But when the particle size is 9 nm, the major neck growth mechanisms are grain-boundary diffusion and surface diffusion. During the sintering process, the fcc structure of the particles transforms to amorphous structure gradually. Besides, the hcp crystal structure forms in the small sized particles because of grain-boundary slid or viscous flow.