Four kinds of plasma-synthesized core-shell type alpha-Fe nanoparticles with various particle diameters, input composition of shell's raw materials, and shell compounds were used to investigate dependence of these nanoparticle parameters on nitridation and magnetic performance. Effects of hydrogen-gas reduction conditions (i.e., temperature and reduction time) prior to nitridation treatment were also investigated in detail. Experimental result showed that the nanoparticle parameters and the hydrogen reduction treatment influenced yield of alpha ''-Fe16N2. Increases in particle diameter and shell amount resulted in the more difficulties in nitridation reaction because of the limitation in nitrogen diffusion phenomena. Changes in shell compound from Al2O3 to SiO2 resulted in the more difficulties in alpha ''-Fe16N2 phase formation. We also found that modification of reduction conditions affect the final product quality. We obtained that by optimization the nanoparticle parameters and the reduction process, the formation of nanoparticles with high yield of alpha ''-Fe16N2 (up to 99%) can be achieved. Finally, we found that for 43-nm core-shell Fe/Al2O3 magnetic nanoparticles containing 10 wt% of Al2O3, the combination of 1.5-h reduction at 300 degrees C and 10-h nitridation at 145 degrees C gave the highest yield of alpha ''-Fe16N2. The best saturation magnetization of 190 emu/g was achieved when using the amount of Al2O3 of 20 wt%. (C) 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.