Abstract: Bio-oil produced from biomass by fast pyrolysis is mainly composed of oxygenated organics and contains high content of water, which makes it unsuitable for direct use as fuel. By developing emulsions from diesel and bio-oil, however, the bio-oil can then be used in compression ignition engines. In this work, the emulsions from diesel and bio-oil are produced by using ultrasonic emulsification method. The physicochemical properties of the emulsions were measured, including density, viscosity, flash point, smoke point, freezing point, and gross heating value. The optimum hydrophilic and lipophilic balance (HLB) value of the emulsifier and the influence of emulsification conditions on the emulsion stability were investigated. The results show that the optimum HLB value of the emulsifier for the emulsification of diesel and bio-oil is about 5.5~6.2; the most stable emulsions are then obtained when the emulsification is conducted at 50℃~60℃ and with the input work of 180J/mL~300J/mL. With the increase of bio-oil content in the emulsions, the density, viscosity, flash point and smoke point of the emulsions are increased, while their freezing point and gross heating value are decreased.