Current research concerning Fe3O4 nanoparticles (NPs) as lubricant additives primarily focuses on the macroscale tribological performance, with insufficient exploration of anti-wear mechanisms at the microscopic scale. In this study, the formation and growth behavior of Fe3O4-based tribofilms was investigated. Oleic acid-modified Fe3O4 NPs dispersed in a polyalphaolefin base oil were tested between ZrO2 balls and GCr15 steel substrates utilizing a reciprocating micro-tribometer. The morphology, microstructure, and chemical composition of tribofilms were meticulously characterized. Results revealed that the tribofilm primarily consisted of cubic Fe3O4 nanocrystals, consistent with initial particles. The organic modification layers of NPs were removed, facilitating direct inter-particle bonding. Growth mechanisms of tribofilms involving tribosintering of Fe3O4 NPs and shear-induced removal were proposed, demonstrating strong dependence on sliding cycles and contact pressure. Under an initial contact pressure of 1.15 GPa, the volume of the tribofilm increased with the number of sliding cycles, eventually reaching a state of saturation. While stress-dependent growth was observed, excessive stress led to wear on the substrate.