The crystal habit and size of the Fe nano-particles prepared by an inert gas evaporation method have been observed with transmission electron microscope. The composition, structure and thickness of the surface oxide film on the Fe nano-particles have been analysed in detail by means of several techniques, such as, X-ray diffraction, electron diffraction, X-ray photo-electron spectrum and Auger electron energy spectrum. A model of the composition and structure for these particles has been given. The results show that the crystalline structure of the Fe nano-particles is consistent with the general α-Fe, i.e. bcc structure. On the surface of these particles, there are double layers of the oxide film with about 3 nm thickness, in which the outside is γ-Fe2O3 of about Inm thickness and the inside is Fe3O4 of about 2 nm thickness. The crystal structure of the two kinds of oxide are of cubic spinel-type. The results also indicate that the Fe nano-particles prepared by the gas evaporation method are very stable in dry air. Though the oxide film is thin, it is quite compact and give good protective effect on the particles.