Ultrasonic emulsification is known to be useful in preparation of nanoemulsions, because use of surfactant can be reduced. Moreover, nanoemuisions with droplet diameters of around 100nm can be prepared by sequential ultrasonic irradiation from low to high frequency. In this study, oil-in-water systems of toluene-water emulsions were prepared by ultrasonic emulsification and mechanical emulsification. Ultrasound emulsification was found to be more suitable than mechanical emulsification with a homogenizer. In addition, a two-step emulsification method was applied to preparation of emulsion. Crude emulsion was prepared mechanically in the first stage, and this was irradiated with ultrasound in the second stage. The droplet size of the emulsion was found to be influenced by the relationship between droplet size of the crude emulsion and ultrasonic frequency in the second stage.