Abstract: The mcrostructure analysis and Study was made on the oxide membrane of the iron powder toner for electrostatic duplicator with optical microscope, X-ray,scanning electron microscopy and Aguer electron energy spectroscopy. The results indicated that when the oxygen content is about 2% a ferroferric oxide membrane of about 2μm thick was formed at the particle surface of iron powder toner and when the oxygen Content of iron powder toner is more than 5%,the thickness of ferroferric oxide membrane at the surface of some iron Powder particles will apparently thicken even if the thickness can increase to 5-8μm. However,it is not a single ferroferric oxide membrane, the ferrous oxide the thickness of which is not homogneous,isformed between the ferroferric oxide membrane and the α-Fe matrix. The presence of FeO will decrease the properties and the service life of the iron powder toner.