The rigid-body displacement at {112} incoherent Σ = 3 grain boundaries in a boron-doped Ni–24at.%Al alloy with the L12structure is determined by transmission electron microscopy using the image matching of fringe contrast in the same g images. From the results, the atomic structure at an incoherent boundary is constructed using a hard-sphere model and interpreted in terms of Ni–Al and Ni–Ni bonding across the boundary.