Anomalously large grains due to secondary grain growth in the ordered Cu3Au thin film were observed by annealing the disordered thin film at 250 °C. No secondary grain growth was observed in the films annealed at 300 and 350 °C. The secondary grains have a preferred orientation of ⟨111⟩. Formation of antiphase domains with sizes larger than the normal grain size was found to accompany the growth of the secondary grains. Transmission electron microscopy showed that the coalescence of ordered grains contributed appreciably to the enlargement of secondary grains and antiphase domains. The driving forces of secondary grain growth and enhanced ordered domain growth are discussed on the basis of surface energy anisotropy and grain boundary migration.