The in situ four-point probe resistivity measurement was used as a main method to study the solid/liquid interfacial characteristics in Ag/Sn/Ag trilayers at temperatures ranging from 150 to 305°C. It is found from the variation of resisitivity that three processes take place on annealing: the dissolution of silver atoms, the diffusion of silver atoms, and the formation of Ag3Sn in liquid tin layer. The first one plays the leading role in the variation of resistivity during annealing process. The apparent diffusivity of silver in liquid tin at 305°C is determined to be 7.3 × 10-17 cm2/s.