[en] Effect of magnetic field on disorder-order transformation in CoPt has been investigated. Single crystals of disordered (A1-type) CoPt were heat-treated at 773 K for 0.5 h under various magnetic fields up to 10 T applied along the [001]_A1direction, and then annealed at 1023 K for 3 h under zero magnetic field. When the magnetic field in the first stage of ordering heat-treatment is larger than 0.5 T, a single variant state of L1₀-type ordered phase with the easy axis (c-axis) being parallel to the applied magnetic field direction is obtained