Abstract: With a view to ever-increasing demand for biomass energy, bio-oils were made from typical microalgae and introduced into CD 15W-40 diesel engine oil as partial alternatives. The lubrication performance of the bio-oils was evaluated with a four-ball friction and wear tester, and their action mechanism was investigated with various modern surface analytical techniques. Results show that as-prepared bio-oils had excellent lubrication performance. Namely, with the increase of bio-oil content in diesel engine oil, the friction coefficient gradually declined, and the wear scar diameter of the steel balls initially declineds but rose later therewith. Particularly, the diesel engine oil containing 10% (mass fraction) of bio-oil had desired tribological properties. Besides, the bio-oil obtained via catalytic liquefaction with La₂O₃ catalyst possessed better lubrication performance than the one obtained via direct liquefaction without catalyst, and the former was able to significantly reduce the friction coefficient and wear scar diameter of the steel balls. In terms of the lubrication mechanism, C- and N-containing organic compounds in the lubricating oil can be adsorbed and deposited on contact surfaces of the steel sliding pair to form a lubricating film, while a protective film composed of Fe₂O₃ and FeN was formed on sliding steel surfaces via tribochemical reactions. As a result, good lubrication and protection efficacy was acquired. The present research, hopefully, is to help to provide reference to broadening the application scope of bio-oil.