Using the sessile-drop method, the wettability and diffusion of the molten Bi–Sn on amorphous and crystalline Fe 78 B 13 Si 9 alloy are studied at isothermal condition, and the interfacial characteristics were investigated by HRSEM and EPMA. The results show that the contact angle of Bi–Sn alloy melts on amorphous substrate is smaller than that of Bi–Sn alloy melts on the crystalline one at 423 K. The interface is continuous, while no intermediate reaction layer formed at the interface of molten Bi–Sn on amorphous and crystalline Fe 78 B 13 Si 9 , respectively. The width of diffusion layer of Bi–Sn alloy melts on the amorphous substrate is thinner than that on the crystalline one under identical conditions.