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The thermal stability of unsupported gold (Au) nanoparticles, containing 140–6708 atoms, has been investigated using molecular dynamics simulation in combination with the modified embedded-atom-method potential. It is found that the melting temperature of the Au nanoparticles decreases drastically with decreasing particle size. The melting temperatures calculated in the present study are in excellent agreement with the previous experimental data. It is further confirmed that the calculated equilibrium shape of the Au nanoparticles is a truncated octahedron bounded by eight (111) and six (100) facets, which can be explained by the anisotropy of the surface energy of Au. On heating, the premelting phenomenon of the surface atoms is apparently observed prior to the melting of the whole particle.