The chemical composition, structure of bio-oil and reaction mechanism of pine sawdust pyrolysis by fluidized bed reactor at 500 degrees C with hot vapor filter (HVF) at temperatures of 350 degrees C, 400 degrees C, 450 degrees C and 500 degrees C were investigated. According to elemental and GPC analysis, the average formula of HVF bio-oils at HVF temperature 350 degrees C, 400 degrees C, 450 degrees C, 500 degrees C and non-HVF bio-oil were expressed as C36.60H54.O-17(15.37), C17.14H25.50O9.07, C15.22H22.53O7.61, C15.80H22.43O8.37 and C15.23H21.47O8.99, respectively. Quantitative NMR result showed that the aromatic carbon fraction and the hydroxyl and alkyl carbons in carbohydrate fraction increased, but the C-O bonds in phenolic-methoxy fraction and the methoxyl-aromatic fraction decreased with the increased HVF temperature. GC x GC-TOFMS analysis showed that the contents of phenols, aldehydes and alcohols in HVF bio-oils and non-HVF bio-oil had a similar increasing tendency. In contrast, ketones, furans and carbohydrates had a similar decreasing tendency. Tentative chemical transformation pathways showed that guaiacol-type compounds with side chains of two carbons or longer were decreased. The Ar-OH bonds in catechol-type were more stable than the Ar-OCH3 bonds when HVF temperature was between 400 degrees C and 500 degrees C. Experimental results showed that hot vapor filter had a positive effect on the composition and structure of bio-oil, and more stable and higher quality bio-oil can be produced. (C) 2016 Elsevier B.V. All rights reserved.