Abstract: The large-scale utilization of biomass pyrolysis oil directly as high-grade energy resources has been quite limited due to its high water content and acidity. The water and acid compounds as a whole, were separated from the pyrolysis oil by molecular distillation. One fraction obtained is rich in acid compounds, and the others are refined biooilⅠ(heavy distillate fractions) and Ⅱ(room temperature condensate ) which have lower water content, weaker acidity and higher heating value. The physical properties of the crude and refined biooil, such as the pH value, heating value and water content were studied. The results indicated that water and acid compounds are separated effectively from the crude bio-oil. The content of carboxylic acids in crude biooil is about 18.85%, while that in the refined biooilⅠand Ⅱ is decreased to 0.96% and 2.2%, respectively.