Interfacial reactions of iron thin films on silicon have been investigated by transmission electron microscopy. FeSi was found to form after 400 °C annealing. Small amount of Fe3Si was detected in samples annealed at 450 to 500 °C. &bgr;‐FeSi2grains were predominant with a few FeSi grains remained in samples annealed at 600 °C. &bgr;‐FeSi2was found to be the dominant phase, whereas &agr;‐FeSi2was predominant in samples annealed at 900–1100 °C in N2ambient and in vacuum, respectively. Two‐step annealings were effective in promoting the growth and improving the quality of epitaxial FeSi2. More uniform growth of epitaxial FeSi2was observed for samples annealed in vacuum than those heat treated in N2ambient. The mechanisms of epitaxial growth and the influence of impurities on the interfacial reactions are discussed.