By comparing field‐ion micrographs of ordered equiatomic PtCo alloy with the surface structure expected from a hemisphere of the established L10 lattice, it is found that the Co atoms are not imaged. This greatly facilitates the domain structure analysis. Using the FIM, 90‐deg oriented domains and antiphase domains are observed at the atomic scale. It is found that, in general, randomly distributed 90‐deg oriented domains and antiphase domains of size about 20–300 Å constitute the ordered PtCo crystal. Additionally, platelets of thickness 20–80 Å, oriented at 90‐deg with respect to the main matrix have been observed. By comparing the ion micrographs of in situ annealed specimens at increasing ordering times, it is concluded that both nucleation and homogeneous rearrangement are responsible for the ordering mechanism at low temperature.