Structure　and　magnetic　properties　were　studied　for　Sm-Fe-N　nanoparticles　prepared　by　surfactant-aid　high　energy　ball　milling.　Magnetic　hardening　mechanism　of　the　particles　was　investigated　by　evaluating　their　initial　magnetization　curves.　The　particles　with　average　size　of　750　nm,　36.5　nm,　and　9.4　nm　exhibit　high　coercivity　of　12.4,　18.7,　and　7.5　kOe,　respectively.　As　particle　size　decreases,　the　magnetization　process　switches　from　pinning　to　nucleation　behavior,　indicating　that　particle　size　plays　a　key　role　in　determining　the　magnetic　hardening　mechanism　of　the　nanoparticles.　(C)　2015　Elsevier　B.V.　All　rights　reserved.