Fe nanoparticles prepared by the arc was used in the present study. It was oxidized by laying in the air and passivated by argon ion bombardment. The microsturcture, oxygen content and oxygen atom position in oxidized Fe nanoparticles were investigated. Furthermore, the variation of oxygen content with time in the passivated Fe nanoparticles and Fe nonparticles was also studied. The results showed that the effective size of oxidized Fe nanoparticles was about 10nm-40nm, and oxidation process was conducted on the surface. On the other hand, the oxygen content in passivated Fe nanoparticles reduced by comparing with that laying in the air for the same time.