A one-pot synthesis of thermally stable core/shell gold nanoparticles (Au-NPs) was developed via surface-initiated atom transfer radical polymerization (ATRP) of n-butyl acrylate (BA) and a dimethacrylate-based cross-linker. The higher reactivity of the cross-linker enabled the formation of a thin cross-linked polymer shell around the surface of the Au-NP before the growth of linear polymer chains from the shell. The cross-linked polymer shell served as a robust protective layer, prevented the dissociation of linear polymer brushes from the surfaces of Au-NPs, and provided the Au-NPs excellent thermal stability at elevated temperature (e.g., 110 degrees C for 24 h). This synthetic method could be easily expanded for preparation of other types of inorganic/polymer nanocomposites with significantly improved stability.