The electron transfer (ET) reaction of ferrocene and ferricyanide at the water-dichloromethane interface, a typical liquid-liquid ET in electrochemistry, was intensively investigated with a three-dimensional free energy surface that fully describes the transport, association, and solvent fluctuation in the ET processes. The calculated free energy surface provides the comprehensive picture of the ET mechanism at the liquid-liquid interface. The present calculation revealed the heterogeneous route of ET that takes place over the interface, rather than the homogeneous one. The present conclusion is found to be consistent with previous results of electrochemical experiments by careful re-examination of the analysis.