Abstract: The effects of oxygen on the magnetic properties of Sm (Co, Fe, Cu, Zr)_z permanent magnets were studied. SEM, EDS and TEM approaches were used to analyze the microstructure of the permanent magnets. The factors that affect the magnetic properties were discussed. The results indicate that the magnetic properties remain excellent and are not affected when the oxygen content is less than 0.32% in mass fraction. The magnetic properties begin to decrease when the oxygen content is between 0.32% and 0.4%, evidently deteriorate when the oxygen content is above 0.4%, and almost vanish when the oxygen content is more than 0.7%. The particle size of the magnets decreases while more Sm₂O₃ begins to separate from the magnets gradually. The main factors that affect the magnetic properties are the increase of non-magnetism phase and the decrease of the useful Sm content of the magnets.