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The growth of β-FeSi2 layers on Si(100) substrates by a cation exchange reaction between Si and molten NaCl–KCl–FeCl2 salts, namely, 5Si(s)+2FeCl2(l)=2β-FeSi2(s)+SiCl4(g), has been investigated. A single-crystal Si(100) substrate was reacted with the molten salt at 1173 K for 1–64 h in Ar or He atmosphere. The grown layers were characterized by X-ray diffraction (XRD) measurement, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). When the FeCl2 concentration in molten salt was as low as 0.02 mol %, a β-FeSi2 single layer was obtained, although the double layer of FeSi/β-FeSi2 formed with a higher FeCl2 concentration of 0.1–1.0 mol %. The β-FeSi2 single layer grown at low FeCl2 concentration had a rough surface structure due to the decrease in driving force caused by the consumption of FeCl2 during the reaction. By annealing a flat double layer of FeSi/β-FeSi2 formed with a higher FeCl2 concentration where the driving force could be kept constant, a flat β-FeSi2 single layer was obtained on the Si(100) substrate.