The structural changes of Ag-Sn films on carbon film have been observed during heating to about 640℃ by using TEM and HVEM. Raising the temperature to 200-250℃, silver and tin react rapidly to form γ phase Ag3Sn. Raising the temperature above 480℃, the γ phase transforms suddenly into laminar β phase. At about 510℃, the β phase film begins to agglomerate. The morphology of the Ag-Sn films on GaAs substrate after heating has been observed by using SEM. It is shown that when the tin content of Ag-Sn films (20-65wt%Sn) is greater and the alloying temperature is higher, the agglomeration of the film is more severe.