The behavior of interface formed by growing this Sm film on the Ｓｉ（１００）２×１ substrate at room temperature has been investigated by cove 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 （０.５ＭＬ≤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. Also we have discussed the interface formation and the interface profile carefully.