Bio-oil, which is produced from pyrolysis and hydrothermal liquefaction of biomass, is becoming one of the most promising renewable and sustainable fuels to replace fossil fuels. It is clean and environmentally benign with some fuel properties similar to petrodiesel. However, the poor fuel properties of bio-oil such as high water content, low pH, and high viscosity challenge its direct application as a fuel in diesel engines. This paper presents the fuel properties of bio-oil from different biomass produced by pyrolysis or hydrothermal liquefaction under varied conditions. Then a comprehensive review and discussion is provided on upgrading of bio-oil by emulsification/microemulsification with diesel and biodiesel or their mixtures. The review finds that most fuel properties of bio-oil can be improved considerably after upgradation, and the resultant emulsion/microemulsion hybrid fuel can be used on diesel engines with reduced emissions of some pollutants. However, there are still many issues remain to be resolved, e.g., the stability and acidity (corrosivity) of the fuel system, before this bio-oil upgrading technology can be scaled up and commercialized. Strategies such as pretreatment of bio-oil, optimization of the use of base oil, surfactant, co-surfactant, and additives, and the selection of emulsion/microemulsion preparation methods should be further investigated for enhancing bio-oil upgrading by emulsification/microemulsification. (C) 2018 Elsevier Ltd. All rights reserved.