Ultrasound was adopted to prepare emulsion fuels between bio-oil and 0# diesel. The effects of ultrasound power and treating time on the stability of emulsion fuels were investigated. Excellent stability with stable time as long as 35 hours was obtained under an ultrasound power of 80W and a treating time of 3 minutes. Malvern nanometer particle size analyzer (Zetasizer Nano S90) was used to study the droplet size of emulsion fuels. The emulsion fuels with smaller droplet size had longer stable time. And the droplet size of the optimal emulsion fuel was around 0.4 um.