The application of ultrasound to-liquid freezing has focused growing attention over the, last few years and its potential seems very promising. In order to make clear droplet freezing assisted by ultrasound, the heat and mass transfer characteristic was studied based on ultrasound theory, penetration theory of mass transfer and energy conservation. The results showed that ultrasound could accelerate mass transfer and make droplet rapid cooling. In the effect of ultrasound, bubble size in the droplet was decreased with ultrasound frequency, and bubble number in the droplet was increased with ultrasound frequency. Mass transfer coefficient of droplet was increased with ultrasound intensity and reduced with ultrasound frequency. For the mass transfer and heat transfer direction were same in the droplet freezing process, the heat transfer was strengthened by mass transfer in the droplet freezing process. Comparing with no ultrasound, droplet temperature with ultrasound was lower 2.0 degrees C-2.5 degrees C after the same time. Hence the ultrasound helps to cool and freeze droplet. (C) 2016 Elsevier Ltd. All rights reserved.