Iron oxide films were prepared by the polymer precursor method with alkaline metal (Li, Na, and K) ion doping. Alkaline metal ions were used to regulate the thermal decomposition process of the cation-citrate complex, that is, the precursor of the film. The spinel iron oxide films were obtained by firing the precursor with the alkaline ion doping [Na/Fe (atomic ratio) ≧ 0.2 and K/Fe (atomic ratio) ≧ 0.2] at 773 K for 5 min in air. The formation mechanism of the spinel iron oxide films was investigated by differential thermal analysis (DTA) and x-ray photoelectron spectroscopy (XPS) measurements. Formation of the carbon-iron oxide complex was observed, and this reduction atmosphere induced the formation of the spinel iron oxide films. This method gave the spinel iron oxide films of which nanostructures are controlled.