Abstract: The activity coefficient of Al in molten Cu decreases with the increasing of electric current which is applied to the liquid alloy of Cu-0.2%Al. To further investigate the mechanism, the quenching experiment of the liquid Al-Cu alloy was conducted. The results show that there is a remarkable change in structure, in which the solute congregates along the direction of the current especially for DC current. The activity coefficient change mechanism of Al in molten Cu-0.2%Al alloy treated by electrical field was discussed. On the basis of this result, a model of the change in solute activity in the liquid alloy treated by electrical field was proposed.