Behavior of magnetic domain structure in the course of magnetization reversal of CoPt crystals with optimum hysteresis properties after annealing under tensile stresses has been studied. Magnetic heterogeneities, which were periodically distributed in {100} and {110} planes have been revealed. An electron-microscopic investigation of the structure of the above CoPt crystals has been performed to show that the crystals are in a nanophase stressed-strained state. A connection between the magnetization-reversal mechanism and characteristic properties of the nanophase state of the crystals is discussed. An important role of magnetoelastic energy in the complicated quasi-periodic distribution of magnetization of the CoPt crystals investigated is emphasized. © Pleiades Publishing, Inc., 2006.