The microstructure evolved after the initial stage of abnormal grain growth (AGG) in Fe–3wt%Si steel was examined using synchrotron X-ray microdiffraction to identify the sub-boundaries, which had been suggested to induce the selective AGG of Goss-oriented grains. The splitting of Laue diffraction peaks caused by sub-boundaries was observed only in Goss grains and not in matrix grains in the entire samples. The sub-boundaries were measured to have very low misorientation angles in the range of 0.1–0.6°. The characteristics and morphologies of sub-boundaries in Goss grains were analyzed based on the splitting directions and positions of Laue diffraction peaks.