We have used STM to investigate the electron tunneling spectroscopy, as well as the tunneling barrier heights,in two kinds of iron oxides prepared differently on iron single crystal surfaces. The experimental results agree reasonably well with that of calculation employing stationary state tunneling method, and show that the iron oxide formed under ambient temperature (type Ⅰ) is mainly consisted of Fe3O4, while the surface processed by flash oxidation (type Ⅱ) should be consisted of high barrier oxides, most likely Fe2O3. Therefore, STM could be used to study various stages of surface oxidation on iron single crystal surfaces. The results aslo indicate that STM could be applied to study a range of metal oxides, and it is possible to investigate the internal structures of certain metal oxides possessing low barrier heights characteristics.