Fe–Cu nanoparticles were synthesized by mechanochemical processing, which utilizes the reaction of FeCl₃ and CuCl₂ with Na during ball milling. Morphologies, structures and magnetic properties of the synthesized nanoparticles were investigated. The crystallite size and mean particle size of the washed nanoparticles after 84 h milling were approximately 9 nm and 50 nm, respectively. During ball milling, the crystallite size remained constant at approximately 14 nm, while the particle size increased. The elemental mapping images of Fe and Cu by imaging filter revealed that the synthesized nanoparticles were a solid solution of the Fe–Cu system. It was seen that numerous hexagonal plates of Fe(OH)₂ exist in the washed nanoparticles. The plates had a few nm in thickness and a few hundreds nm in diameter. The coercivity of the synthesized Fe–Cu nanoparticles after 48 h milling was as high as 33.5 kA/m. This is because the particle is close to the critical size for a single magnetic domain of α-Fe.