Silver sulfide (Ag2S) nanoparticles were prepared by in-situ method, and displayed the small size and good dispersibility in the lubricating coatings. A simple strategy was provided to regulate the size of the Ag2S nanoparticles. The influences of Ag2S nanoparticles and their sizes on the micro-hardness and tribological behaviours of the lubricating coatings were investigated, and the corresponding tribological mechanisms were discussed. Detailed investigation showed that the in-situ synthesized Ag2S nanoparticles dominated by smaller-size and narrower size distribution increased the micro-hardness by 53.39% and reduced wear rate by 26.41%. The friction coefficient was independent to their size and size distribution. The superior tribological performance was attributed to the synergetic roles of the stronger micro-hardness and the high load-carrying tribofilm.