Air stable Co3Fe7-CoFe2O4 nanoparticles have been synthesized via one-step flame spray pyrolysis of a mixture of Fe/Co precursor solution under stronger reducing atmosphere. The as-synthesized nanoparticles with diameters of 20-80 nm showed a typical core shell structure and high stability for being one month in air, whose metallic Co3Fe7 cores were protected against oxidation by a surface shell of about 2-4 nm cobalt iron oxide (CoFe2O4). The ratio of metallic Fe/Co alloy nanoparticles was 7:3. The alloy nanoparticles exhibited enhanced saturation magnetization (126.1 emu/g), compared with flame sprayed iron nanoparticles with the same conditions. The formation process of metallic alloy nanoparticles with core-shell structure was investigated, which included three stages: flame combustion, reducing, and surface oxidation during the flame process. It is reckoned that such a continuous production approach is an effective way to produce the stable Co3Fe7 alloy nanoparticles with high saturation magnetization.