In this paper, emulsion fuels from crude bio-oil and its molecular distillation fractions were prepared by single ultrasonic and ultrasonic-mechanical emulsification. The results showed that the emulsions from crude bio-oil and 0# diesel had the best stability, with a stability time of 31 days. The stability time of the bio-oil middle fraction emulsion was 216 min, while that of the bio-oil heavy fraction emulsion was as low as 14 min. The particle size distributions of the emulsions were analyzed by a Malvern particle size analyzer. Emulsion fuels made from crude bio-oil and the bio-oil middle fraction had a smaller droplet size of 8-60 nm. In contrast, the emulsion fuel from the alcohol-diluted bio-oil heavy fraction had much larger droplet size, reaching up to 6000 nm. Compared with single ultrasonic emulsification, ultrasonic-mechanical emulsification had positive effects on increasing the stability time of the crude bio-oil emulsion, but negative effects on the stability time of the bio-oil middle and heavy fraction emulsions. To explain the above differences, spherical shell mechanism models were proposed according to the composition properties of the crude bio-oil and its fractions. (C) 2014 Elsevier Ltd. All rights reserved.