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Several authors have emphasized that aluminum nitride in iron and iron alloys not only lowers the transition temperature but also suppresses the rise of the transition temperature due to irradiation. To clarify the mechanism of the effect of aluminum nitride, structural changes after irradiation and annealing are observed by means of internal friction and electron microscopy. Materials investigated are iron, containing 0.05 wt% Al and 0.01 wt% N with <1 ppm C and a few ppm 0, and iron, containing 0.02 wt% Al and 0.005 wt% N with 10 ppm C and 20 ppm 0. The materials are annealed at 750 C before irradiation and irradiated in the JRR-2 at temperatures of 60 C and up to 1.2×1019 n/cm2, E > 1 MeV. Two new peaks in internal friction are found at ∼125 C and ∼155 C after irradiation. Besides these changes a new type of precipitate is observed by electron microscopy. These precipitates are thin, doughnut shaped disks with a diameter of 0.3 μm and are considered to be complex defects of irradiation induced point defects, nitrogen, and aluminum atoms. The 125 C peak of internal friction disappears after annealing at 250 C for 30 min; the 150 C peak disappears after annealing at 150 C for 30 min or aging at room temperature for about half a year. Resolution of free nitrogen is observed, but precipitates do not disappear even after annealing at 350 C for 30 min. The precipitates suppress the formation of irradiation induced complex nitrogen defects and irradiation hardening because of the strong effect of aluminum on nitrogen atoms.