Thin films of iron oxides and nitrides containing various amount of oxygen and nitrogen were prepared by an R.F. reactive sputtering. Changing the partial pressure of reactive gases (oxygen and nitrogen) from 5×10-6 to 5×10-3 Torr in argon gas (total pressure : PO2 (or PN2) + PAr = 5×10-3 Torr), thin films were deposited onto the glass substrates.The deposition rate and the properties of the films such as electrical resistivity, crystal structure and chemical composition were examined with the partial pressure of reactive gases. A critical pressure at which the deposition rate of the films decreased drastically was observed by the reactive sputtering in argon plasma at the partial pressure of oxygen (2×10-4 Torr) and nitrogen (4×10-4 Torr). In both cases of the Fe-O2 and Fe-N2 reactive sputtering, the structures of the deposited films were observed to begin changing in the region of the critical pressures, namely, FeO, Fe2O3 and Fe3O4 in the Fe-O2 system and Fe2N, Fe3N and Fe4N in the Fe-N2 system were observed over the critical pressures.Reactive sputtering experiments under helium gas instead of argon gas were also carried out in order to reveal the interaction between the plasma gases with different mass and the target surfaces in the sputtering process.