In order to enhance the condensation efficiency during molecular distillation experiment, crude bio-oil was subjected to pretreatment consisting of traditional vacuum distillation. Most of the water in the crude bio-oil was removed first, and then this was used as the feed bio-oil for two molecular distillation processes. Four fractions were obtained under the operation parameters of 80 degrees C, 1600 Pa and 80 degrees C, 340 Pa. The yields of distilled fractions 1 and 2 were 26.36% (w/w) and 22.58% (w/w), respectively. The distilled fractions were rich in low molecular weight carboxylic acids and ketones, so they had better flow ability than the residual fractions. The water in the residual fractions was almost completely removed and the heating values of residual fractions 1 and 2 reached 21.29 MJ/kg and 22.34 MJ/kg. The integrated separation characteristic of bio-oil was explored by a macro-level evaluation model. It based on the distribution properties of functional groups and gave information that carboxylic acids, ketones and furans had better distillation characteristics than aldehydes, phenols and sugars. A self-defined separation factor beta was used in the micro-level evaluation model to specify the separation characteristics of 15 typical compounds. It indicated that acetic acid and 2-propanone, 1-hydroxy had a high separation factor of 0.9 while 5-hydroxymethyl, furaldehyde and 1,2-benzenediol had a separation factor as low as 0.04. (C) 2010 Elsevier B.V. All rights reserved.