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Nickel (Ni) nanoparticles can be enriched on the surface of iron-based frictional pairs, which provides the possibility to get rid of the competitive adsorption between the polar species of vegetable oil and the surface-active nano-additives thereon. In this paper, nickel acetylacetonate was used as a precursor to in-situ synthesize nickel nanoparticles with an average diameter of about 12 nm in rapeseed oil (RO) as the reducing agent, surface modifier, and solvent as well. The tribological properties of the as-synthesized Ni nanoparticles were evaluated with a four-ball tribometer, and their tribomechanism was investigated based on the characterizations of the tribofilm on rubbed steel surfaces by the scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). It was found that the Ni nanoparticles in-situ prepared in the RO with a mass fraction of 0.3% can reduce the wear scar diameter (WSD) of the steel ball by 36%. This is because, on the one hand, the Ni nanoparticles are adsorbed on the rubbed steel surfaces to repair or fill up the micro-pits and grooves thereon. On the other hand, Ni nanoparticles participate in tribochemical reactions with atmospheric O and steel substrate to form the tribochemical reaction film on the rubbed steel surfaces with the assistance of friction-induced heat and applied normal load. In addition, an amorphous carbon film is formed on the rubbed surface via the carbonization of base oil under the catalysis of Ni nanoparticles. The adsorbed Ni layer, the tribochemical reaction film, and the carbon layer comprise a composite tribofilm composed of amorphous carbon, polar fatty acid, metallic nickel, iron oxides, and nickel oxides on the rubbed steel surfaces, which contributes to significantly improving the antiwear ability and load-carrying capacity of the RO for the steel–steel sliding pair. [ABSTRACT FROM AUTHOR]