Metastable α"-Fe 16 N 2 is considered to be a good candidate as a semi-hard magnetic material with high saturation magnetization. To obtain a higher coercivity H c , it is necessary to improve the magnetocrystalline anisotropy of α"-Fe 16 N 2 . For this purpose, many theoretical calculations based on substitution of Fe with other elements (i.e., Co, Ni, Mn, Al) have been performed. In this study, to investigate the effect of substitution of Fe by other elements M (M = Al, V, Cr, Mn, Ni), the synthesis of the α"-(Fe 0.95 M 0.05 ) 16 N 2 nanoparticles by reduction of α-(Fe 0.95 M 0.05 )OOH as the starting material and subsequent nitridation was undertaken. The α-(Fe 0.95 Al 0.05 )OOH nanoparticles used as the starting material were spherical in shape and the particle size was significantly smaller than that for α-FeOOH and that for α-Fe 0.95 Al 0.05 obtained by reduction, which was about 25 nm. The α"-(Fe 0.95 M 0.05 ) 16 N 2 nanoparticles appeared only for M = Al and V. The result suggested that the experimental conditions whereby α"-(Fe 0.95 M 0.05 ) 16 N 2 was produced were controlled by the crystallite diameter (Dc) for the α-Fe 0.95 M 0.05 nanoparticles which was <; 50 nm before nitridation irrespective of the nature of element M. This finding is critical for preparing materials like α"-(Fe,M) 16 N 2 where the nitrogen atoms are introduced at the interstitial site and where the distorted crystal structure is controlled by the limitation of the crystallite diameter for α-(Fe,M) as the precursor. Al was effective in preventing sintering during reduction, which led to a small Dc value being obtained. The highest Hc value for the α"-(Fe 0.95 Al 0.05 ) 16 N 2 nanoparticles was 2200 Oe for reduction at 450 °C for 4 h and subsequent nitridation at 160 °C for 20 h.