Abstract: High ₁Hc (>30KOe) has been obtained in Sm (Co Cu Fe Zr)_7.4 magnet after aging at 850 ℃ for a long period of time. Coercivity, initial magnetization curves, magnetic domain patterns, microstructures and microstres of the diffrent aging time at 850 ℃ are Studied using magnetic measurement, Kell technical and transmisson electron microscopy. The action of "Z" phase, zonal structure and grain boundary precipitates on domain walls are discussed and a machanism of heterogeneous pinning at the high coercivity is suggested.