The synthesis of Sm2Fe17N3 magnetic particles has been vigorously studied because of their superior magnetic properties. Here, we developed a novel method to form Sm2Fe17N3 magnetic particles with relatively high coercivity (1.3 T), which were formed from Fe3O4-SmOx core-shell nanoparticles using a reduction-diffusion process, followed by in-situ nitridation. This method would provide not only smaller Sm2Fe17N3 particles with higher coercivity but also other rare-earth containing materials.