The development of a high-performance nanoparticle (NP) additive for lubricating oil is a research hotspot for the tribology and engineering areas. In this study, the concept of a novel liquid nano-additive has been proposed based on the emergence of Ga-based liquid metals (GLMs), which display excellent extreme-pressure and high-temperature lubricity. Herein, the liquid NPs (designated as GLM-NP/C12) were prepared from a GLM droplet through the ultrasonic method, modified with 1-dodecanethiol, and are mainly distributed at 286 +/- 21 nm. They have a core-shell structure with liquid-state GLM on the inside, and gallium oxide and a self-assembled alkylthiolate monolayer on the outside. In terms of the tribological performance, GLM-NP/C12s have awonderful dispersion-stability in PAO10 oil, and provide excellent anti-adhesion, friction-reducing, and wear-resistance properties. When the additiveconcentration was 0.17 wt% in PAO10, the friction coefficient was reduced by 39% and the wear rate was reduced by 93% compared to those lubricated by the neat PAO10. This kind of liquid nano-additive has the advantages of easy preparation, internal composition regulation and recyclability, compared to conventional solid NPs. In addition, the liquid NPs were readily introduced into the frictional interfaces. More generally, the optimal additive concentration of the liquid NPs was much lower than that of the solid NPs. This observation has important implications for understanding the differences of the lubrication mechanisms between the solid and liquid nano-additives, and may provide a new design method and strategy of nano-additives for lubricating oil.