Microstructures and magnetic domain structures of precipitation-hardened Sm(Co 0:720 Fe 0:200 Cu 0:055 Zr 0:025 ) 7:5 permanent magnets obtained by various heat treatments are investigated by transmission electron microscopy (TEM). It is found that Cu atoms gradually segregate into SmCo 5 phase with the increase in aging time. The domain walls in the solution-treated, 6-h isothermal-aged magnets are straight, while those in the step-aged magnet are zigzag shaped along the cell boundaries of the SmCo 5 phases (1:5 H phases). In the demagnetized state of the step-aged magnet, it is found that the domain wall is located almost on the 1:5 H cell boundary phase containing Cu atoms, where the distribution of lines of magnetic flux strongly deviates from the axis of easy magnetization, particularly near the zigzag domain wall, and the lines of magnetic flux flow symmetrically along the center of the 1:5 H cell boundary phase. In the remanent state of the step-aged magnet, it is confirmed that domain walls are strongly pinned almost to the 1:5 H cell boundary phase containing Cu atoms, eventually resulting in a high coercivity in a Sm