Fe-Co nanoparticles were synthesized by the thermal decomposition of a mixed precursor of Fe(CO) 5 and Co 2(CO) 8. As-synthesized Fe-Co nanoparticles formed clusters 20 nm in diameter consisting of many primary 5-nm nanoparticles. The saturation magnetization of as-synthesized Fe-Co nanoparticles was about 140 emu/g net. This was only 60 of the saturation magnetization of bulk Fe 70Co 30, due to the presence of a surfactant and the small grain size within the primary nanoparticles of less than 20 nm. During post-annealing at 250°C, the primary NPs sintered, and the nanoparticle clusters grew into a single crystal by atomic diffusion. As a result, the saturation magnetization reached a maximum of 212 emu/g net for Fe : Co 68 : 32, and varied with chemical composition in a relationship similar to the Slater-Pauling curve.