Abstract The behavior of interface formed by growing thin Sm film on the Si(100)2×1 substrate at room temperature and its temperature evolution has been investigated by core-level and valence-band photoemission using synchrotron radiation. The experimental results show the existence of distinct stages corresponding to chemisorption and agglomeration of Sm atoms(coverage ϑ<0.5ML), reactive interdiffusion(0.5< ϑ<4–6ML), and growth of metallic Sm. Compared to Si(111)7×7 the reactivity of Sm on the Si(100)2×1 substrate is enhanced and a greater tendency for interdiffusion of Sm and Si is observed. Like on Si(111), a multiphase interface is formed but less stable. After being annealed, this phase disappears gradually. When the annealing temperature gets up to 1000 °C, an original 2×1 LEED patterns appear again on the surface. Together with a model, the interface formation and the interface profile is discussed in detail in the text.