Abstract: The bio-oil from fast pyrolysis of biomass has high oxygen content, low heating value and chemical instability, which affect the application of bio-oil. The upgrading of bio-oil has been required in order to improve the quality of bio-oil. The upgrading of bio-oil, including hydrogenation, catalytic cracking, vapor-phase catalytic pyrolysis, steam reforming, and emulsification has been reviewed. Research trends in bio-oil upgrading have been discussed. It is hoped that this review can aid the application of bio-oil.