a00-Fe16N2 nanoparticles (NPs) have been identified as one of the most promising rare-earth-free magnetic materials and synthesized with high purity in laboratory investigation recently. This study reports characteristics and magnetic performance of spindle shaped core–shell a00-Fe16N2/Al2O3 NPs successfully synthesized from hematite NPs. A two-step reaction process, i.e., hydrogen reduction and ammonia nitridation treatments, was employed to produce the a00-Fe16N2 phase (making up to 99 wt.% of the core content). The high-purity NPs exhibited saturation magnetization and coercivity of 186 emu/g and 2.2 kOe, respectively, at 300 K. The obtained a00-Fe16N2/Al2O3 NPs possessed a spindle shape with an internal pore structure. Their porosity increased from 10% to 50–60% during two-step synthesis process, however, at the expense of crystallite size. The combined use of the present synthesis method and hematite NP enables reduction in the synthesis reaction time, as well as the fabrication of NPs with excellent magnetic performance. Furthermore, the present results suggest the potential of rare-earth-free magnetic core–shell a00-Fe16N2/Al2O3 NPs in industrial applications.