A (&sqrt;3×&sqrt;3)B structure was found to be formed on a (5×5) GexSi1−x/Si (111) surface on which Ga or Sn atoms did not form any superstructures. The critical B coverage at which a (7×7) pattern disappeared and only a (&sqrt;3×&sqrt;3) pattern was visible increased as the fraction (x) of Ge in the substrate layer increased. A Si epitaxial overlayer was grown on the (&sqrt;3×&sqrt;3)B/50 A˚ Ge0.4Si0.6/Si (111) structure at a growth temperature of 300 °C. The observed (−2/3,4/3) reflection intensity in grazing x‐ray diffraction was 50 times larger than that of a Si epitaxial layer grown on a (&sqrt;3×&sqrt;3)B/Si (111) structure under the same condition. On a GexSi1−xsubstrate, the B(&sqrt;3×&sqrt;3) structure is well preserved at the interface probably because of relief of the interface strain that results from the small size of the boron atom.