α″ -Fe16N2 has been suggested as a promising candidate for future rare-earth-free magnets. In this paper, we report to use high magnetic field (9T) assisted post-annealing process to enhance the Fe16N2 phase formation in FeN bulk rod samples during the α′ → α″ phase transformation and thus improve its magnetic properties. It was found by X-ray Diffraction measurement that the volume ratio of Fe16N2 phase was increased up to 22%, which corresponds to an increase in the amount of transformation from α′ → α″ up to 78%. Also, the saturation magnetization (Ms) of the prepared FeN rod sample was increased to 227emu/g with its coercivity up to 376Oe at room temperature. A working mechanism for the high field assisted post-annealing process was presented.