Gas phase synthesis of large directly ordered L 10 FePt nanoparticles was studied. Simultaneous control of the chemical ordering and the size of the FePt nanoparticle was successfully achieved. It was found that the chemical ordering of the FePt nanoparticles was mainly influenced by the energy conditions (thermal environments at nucleation and growth regions), which could be adjusted by varying the process parameters including the sputtering current density, the Ar gas pressure, etc. The sizes of ordered FePt nanoparticles were more related to the Fe and Pt atoms' density at the initial stage (close to target surface). Multiply twinned structures were observed in L 10 FePt nanoparticles with 12 and 17 nm mean sizes, but were absent in L 10 FePt nanoparticles with 6 nm mean size, which caused the relatively low coercivity of large L 10 FePt nanoparticles.