Abstract: A mathematical relation is derived for mass transfer coefficient at phase interface evolving ultrasonic gas cavities based on the characteristic of fluid flow surrounding ultrasonic gas cavities and the penetration theory of mass transfer,combining with the principles of hydrokinetics.This shows that the ultrasonic power can accelerate mass transfer,and the mass transfer coefficient induced by ultrasonic gas cavities y is not only approximately proportion to the intensity of ultrasonic,but also to the square root of the diffusion coefficient and the volume rate of gas caused by ultrasonic gas cavities,and meanwhile related to the factors such as bubble size R_d,interface area S′ and so on..The mathematical relation is checked with literature date.It shows that the mathematical relation is consistent with the results of experiments,and can better describe the behavior of mass transfer induced by ultrasonic gas cavity in liquid.So,a theoretical basis of using ultrasonic to strengthen mass transfer process is provided by the mathematical relation.