Bio-oil can't be directly used as fuel due to its deteriorate properties. Here, an efficient catalytic upgrading process for the bio-oil, including esterification, hydrogenation, hydrodeoxygenation and depolymerization, is proposed with multifunctional catalyst Ni/SiO2-ZrO2 and biomass-derived solvent ethanol. Results showed that esters, alcohols, phenolics, and cyclo-ketones were the main components in the upgraded bio-oil while aldehydes were removed completely via catalytic hydrogenation and acids were removed by catalytic esterification with supercritical ethanol. The pH value of upgraded bio-oil rose drastically from 2.38 to 5.24, and the high heating value increased to 24.4 MJ kg(-1). Comparison characterization on the upgraded and crude bio-oil using FT-IR, GPC (Gel permeation chromatography) and C-13 NMR (Nuclear Magnetic Resonance) demonstrated that lignin-derived oligomers contained in crude bio-oil were further depolymerized over Ni/SiO2-ZrO2 catalyst. The improved properties suggest that the upgraded bio-oil is more suitable to be used as boiler fuel. Furthermore, the loss of carbon is negligible because formation of coke is suppressed during the upgrading process. (C) 2015 Elsevier Ltd. All rights reserved.