Abstract: The bio-oil from fast pyrolysis of biomass has chemical instability and high viscosity and is difficult to be emulsifid with 0# diesel. Surrounding these problems, an emulsification process, the bio-oil aqueous solution was emulsified with 0# diesel, is suggested. Four common emulsifiers and one co-emulsifier were selected and remixed to two new emulsifiers. Under certain conditions, the effect of two remixed emulsifiers, the mass ratios of remixed emulsifiers and the ultrasonic time on emulsification results were studied. The results show that the six emulsions are not broken within 30d. Compared with characteristics of 0# diesel, its density and heat value are almost the same, water content is below 3%, kinematical viscosity increases about 40%, and pH value decreases about 50%. Factor analysis shows that mass ratio of bio-oil aqueous solution to diesel and different types of bio-oil aqueous solutions are most important to influence on the emulsification. The emulsification mechanism was preliminarily discussed. Polar compounds such as water, aldehyde, acid and ketone from bio-oil aqueous solution were wrapped in W/O emulsion droplet. The compounds such as ethyl acetate and aromatic compounds were solubilized in the micelles of the nonionic emulsifiers. The thermodynamic analysis on emulsification shows that entropy increase of ultrasonic emulsification process is more than that of static biooil, and the emulsions tend to be equilibrium state.