Abstract: As a novel alternative fuel for diesel oil, bio-oil was upgraded by emulsification technology and the tribological performances of bio-oil and upgrade ones were tested on a cylinder liner-piston ring tribometer. The surface profilometer, scanning electron microscopy and X-ray photoelectron spectroscopy were used to analyze the micro-morphology and the valence states of elements on the rubbing surface and the tribological mechanisms were also discussed. The results show that the bio-oil from Chlorella had better friction-reducing and anti-wear properties than those of bio-oil from rice husk. The performances of the bio-oil can be improved significantly by the emulsification method. The lubrication mechanism was ascribed to the formation of lubricative films from adsorption, extrusion, and frictional deposition of the bio-oil organics on the cylinder surface, the rolling of micro-balls caused by local frictional melting and protective oxidation film of Fe₂O₃ and FeOOH on the friction surface during the friction process. Moreover, the bio-oil from Chlorella formed a nitrogen-contained organic film on the frictional surface, which was also an important reason for the better tribological performances.