There is provided an iron oxide magnetic nanoparticle powder having a ferromagnetic property even if the particles have an average particle size of 15 nm or less, preferably 10 nm or less, and a method of producing the same, an iron oxide magnetic nanoparticle thin film containing the iron oxide magnetic nanoparticle powder and a method of producing the same, wherein the iron oxide magnetic nanoparticles having an ε-Fe2O3 single phase, having the average particle size of 15 nm or less, and further 10 nm or less, are generated by using β-FeO(OH) (iron oxide hydroxide) nanoparticles as a starting material, and coating the (iron oxide hydroxide) nanoparticles with silicon oxide, and applying heat treatment thereto under an atmospheric air, and further the iron oxide magnetic nanoparticle thin film is obtained by using the iron oxide magnetic nanoparticles.