Co2FeSn Heusler nanoparticles with the particle size of 5 12 nm were synthesized by using pre-synthesized Co2Fe and Sn nanoparticles in high boiling-point solvents, wherein the molten Sn diffused into Co2Fe nanoparticles to form Co2FeSn nanoparticles. The obtained Co2FeSn nanoparticles show a high magnetic moment of 5.2 mu(B) at 5K, which is comparable with that of most of the bulk Co(2)YZ Heusler compounds. This low-temperature diffusion method is believed to have potential applications in synthesizing other Heusler nanoparticles.