Abstract: Experiment research concerning the mass transfer of flowing boundary layer on a liquid-liquid interface was performed to discuss the mass transfer on the molten slag-metal interface in metallurgical reactor. The mass transfer phenomenon of sodium benzoate on the oil-water interface was analyzed in the condition of keeping oil-water interface flat. The concentration change of sodium benzoate with time in water was measured by conductivity meter when the volume flow of water varied from 0.44 to 1.60 m³/h. A dimensionless number equation with respect to the mass transfer coefficient of flowing boundary layer on the liquid-liquid interface was obtained. Then, a dimensionless number equation between the mass transfer coefficient and viscosity as well as surface tension was given tentatively. The results were compared with the mass transfer of boundary layer in the case of fluid flowing through a flat.