Abstract: The tribological properties of ultrafine surface-coated serpentine powder as a mineral oil additive were evaluated under varying speed and load. The morphology and element distribution of worn steel surfaces lubricated by base oil and base oil containing serpentine additive were analyzed using scanning electron microscope and energy dispersive spectrometer. The micro-mechanical properties and scratch resistance of the normal worn surface (lubricated by base oil) and the tribofilm (lubricated by base oil containing serpentine additive) were analyzed using a nano-indentation tester. Furthermore, Stribeck tests for point contacts were carried out to compare the tribological properties of the oil/normal worn surface system and the oil/tribofilm system in different lubrication regimes. Results show that serpentine additive gives birth to a porous oxide tribofilm with a high hardness and a Young’s modulus similar to that of steel. Besides, embedded oxide particles can further improve nano-mechanical properties of the tribofilm, while porous tribofilm can act as an oil reservoir to retain oil and capture wear particles simultaneously. As a result, the tribofilm possesses excellent tribological properties under boundary and mixed lubrication conditions.