The metastable <inline-formula> <tex-math notation="LaTeX">\alpha ^{\prime \prime } </tex-math></inline-formula>-Fe 16 N 2 is considered to be a candidate for rare earth-free semi-hard magnetic materials. However, it is necessary to improve the magnetocrystalline anisotropy of <inline-formula> <tex-math notation="LaTeX">\alpha ^{\prime \prime } </tex-math></inline-formula>-Fe 16 N 2 to obtain higher coercivity, <inline-formula> <tex-math notation="LaTeX">H_{c} </tex-math></inline-formula>. Many theoretical calculations and thin-film experiments have been performed by the substitution of Fe with elements such as Co, Ni, Mn, and Al. For example, Co is suggested to increase the magnetocrystalline anisotropy. The nitridation of <inline-formula> <tex-math notation="LaTeX">\alpha </tex-math></inline-formula>-Fe nanoparticles produced by the reduction of an iron hydroxide such as <inline-formula> <tex-math notation="LaTeX">\alpha </tex-math></inline-formula>-FeOOH is well known as one method for the synthesis of <inline-formula> <tex-math notation="LaTeX">\alpha ^{\prime \prime } </tex-math></inline-formula>-Fe 16 N 2 nanoparticles. We have previously reported the influence of Co or Al substitution for Fe by the synthesis of <inline-formula> <tex-math notation="LaTeX">\alpha ^{\prime \prime } </tex-math></inline-formula>-(Fe, Co) 16 N 2 or <inline-formula> <tex-math notation="LaTeX">\alpha ^{\prime \prime } </tex-math></inline-formula>-(Fe, Al) 16 N 2 nanoparticles, and revealed that Co substitution results in very narrow limits of the temperature regions for reduction and nitridation, while Al substitution results in wider temperature regions. Here, we attempt the synthesis of <inline-formula> <tex-math notation="LaTeX">\alpha ^{\prime \prime } </tex-math></inline-formula>-(Fe 0.95 Co 0.02 Al 0.03 ) 16 N 2 nanoparticles by hydrogen reduction of <inline-formula> <tex-math notation="LaTeX">\alpha </tex-math></inline-formula>-(Fe 0.95 Co 0.02 Al 0.03 )OOH as a starting material and subsequent nitridation to investigate the complex effect of Fe substitution by Co and Al.