The energy‐focused time‐of‐flight atom‐probe field‐ion microscope (FIM) is employed to study surface segregation of a Ni‐Cu alloy in nitrogen and oxygen. In nitrogen, in addition to the normal enrichment of Cu to the top surface layer as in the vacuum annealing case, highly Cu‐rich regions are formed near plane edges of the top surface layer. In oxygen, no Cu enrichment at the surface is found. Both the composition depth profiles and the spatial distributions of alloy species within an atomic layer can be obtained by the atom‐probe (FIM) with depth and spatial resolution of a few angstroms.