An iron nitride magnetic powder consisting primarily of Fe.sub.16N.sub.2 phase whose average particle diameter determined by particle size measurement using a TEM micrograph is 20 nm or less and whose geometric standard deviation of the particle diameter is 1.4 or less. The iron nitride magnetic powder can be obtained by a method of producing an iron nitride magnetic powder consisting primarily of Fe.sub.16N.sub.2 phase includes a step of, at the time of producing an iron nitride magnetic powder consisting primarily of Fe.sub.16N.sub.2 phase by subjecting a reduced powder obtained by reduction of iron oxide to ammonia treatment, using goethite containing Al in solid solution as the iron oxide.