The boron‐induced ( 7/8 × 7/8 )R30° reconstruction at the Si interface has been investigated by grazing incidence x‐ray diffraction. The in‐plane projected struture is found from the structure factors near zero perpendicular momentum transfer. At thea‐Si/Si (111) interface, boron atoms at 1/3 ML are substituted for silicon atoms, thus forming a ( 7/8 × 7/8 )R30° lattice and the boronsilicon bond is contracted compared with the siliconsilicon bond. Even at the interface between a solid phase epitaxial Si (111) layer and a Si (111) substrate, the boron‐induced ( 7/8 × 7/8 )R30° reconstruction has also been observed and the structure is similar to that observed at thea‐Si/Si (111) interface.