Metallic iron(Fe) nanoparticles(NPs) with a typical core-shell structure have been prepared by a simple and continuous flame spray pyrolysis(FSP) method,which are stabilized by the corresponding Fe_3O_4 shell with a thickness of 4-6 nm.The size of metallic Fe cores is about 30-80 nm.The core-shell structured iron NPs show an air stability as long as one month as a result of the protection of oxide shell.Through the control of the residence time of materials in flame and flame atmosphere,metallic Fe and iron oxides are obtained,showing a better external magnetic field responsibility.It is concluded that the evolution of morphology and composition of flame-made magnetic NPs could be attributed to the competition mechanism between reduction and oxidation reactions of in situ flame combustion,which offers more choices and better effective design strategy for the synthesis of advanced functional materials via FSP techniques. Metallic iron (Fe) nanoparticles (NPs) with a typical core-shell structure have been prepared by a simple and continuous flame spray pyrolysis (FSP) method, which are stabilized by the corresponding Fe_3O_4 shell with a thickness of 4-6 nm. size of metallic Fe cores is about 30-80 nm. The core-shell structured iron NPs show an air stability as long as one month as a result of the protection of oxide shells. Through the control of the residence time of materials in flame and flame atmosphere, metallic Fe and iron oxides are obtained, showing a better external magnetic field responsibility. It is concluded that the evolution of morphology and composition of flame-made magnetic NPs could be attributed to the competition mechanism between reduction and oxidation reactions of in situ flame combustion, which offers more choices and better effective design strategy for the synthesis of advanced functional materials via FSP techniques.