Abstract: The friction and wear behaviors of different oils (micro-emulsified bio-oil and diesel fuel) under reciprocating sliding condition by a tribometer of piston ring-cylinder liner were investigated. The morphology of worn surface and the content of elements on worn surface were analyzed by scanning electron microscopy and energy dispersive spectroscopy. The chemical states of several typical elements on the worn surface were examined by means of X-ray photoelectron spectroscopy. The mechanisms of friction and wear was explored. The results show that the friction coefficient and wear of piston ring and cylinder liner increased with the reciprocating frequency under lubrication of micro-emulsified bio-oil. The friction reduction of micro-emulsified bio-oil was better than that of diesel fuel while wear resistance of piston ring and cylinder liner in micro-emulsified bio-oil was worse than that in diesel fuel. The mechanism of friction and wear was attributed to polar group in the micro-emulsified bio-oil which easily adsorbed on the rubbing surface and led to better lubrication. However, the corrosion role on the rubbing surface was enhanced and intensified the debris abrasion. Fe₂O₃ acted as abrasive particle and increased the wear.