Abstract: Esterification of acetic acid (AC), propanoic acid (PA) and a mixed acid (AC + PA + acrylic acid) as well as real bio-oil with ethanol under supercritical CO₂ (scCO₂) conditions were investigated. It was found that these carboxylic acids could afford much higher conversions under scCO₂ condition in comparison with esterification at atmosphere pressure, revealing the promoting effect of scCO₂ as a result of that the formed esters are prone to be phase-transferred into the scCO₂ phase. Since raising pressure at constant temperature can enhance the dissolving power of scCO₂, it was observed that esterification conversion increased with pressure. The results esterifying these acids separately showed significant difference in their conversions, whereas simultaneous esterification of the mixed acid mixture afforded very close conversions, suggesting that a transesterification mechanism might occur in the case of simultaneous esterification under scCO₂. The appropriate operation conditions for upgrading bio-oil by scCO₂ extractive esterification were determined as at 80℃ and 28.0MPa for 3.0h. Under this condition, the total acid conversion could reach up to 86.78%. pH value for the upgraded bio-oil increased from 3.78 to 5.11. The total volatile components in the upgraded bio-oil approached up to 100% at 140℃.