Abstract: The coalescence process of a small drop on a plane liquid-liquid interface in pure liquid-liquid systems has been studied.A model for the dynamics of the thinning film was developed with the interfacial mobility taken into account.London-van der Waals force in thin liquid film was recognized.The model predicted the coalescence time,which is the time required for the thinning and rupture of the liquid film,given the radius of the drop and the required physical properties of a liquid-liquid system.The predicted result was in good agreement with previous experimental results.