Trimodal aluminum (Al) metal-matrix-composites (MMCs), consisting of B4C particulates, a nanocrystalline Al (NC-Al) phase, and a coarse-grain Al phase (CG-Al), has been fabricated. These MMCs exhibits extremely high compressive strength and tailorable ductility. Excellent thermal stability of NC-Al grains and high strength has been attributed partially to the nitrogen present within the trimodal Al MMCs, which is introduced during the cryomilling process in liquid nitrogen. This paper describes an investigation into the concentration and constituents of nitrogen within the trimodal Al MMCs. The structure of nitrogen-containing dispersoids was examined by analytical transmission electron microscopy (TEM), and secondary ion mass spectrometry (SIMS) was employed to determine the total concentration of nitrogen. The nitrogen concentration increased linearly with an increase in cryomilling time up to 24 h. Both crystalline and amorphous aluminum nitrides with very fine size, down to 5 nm, as dispersoids, have been observed by analytical TEM. Correlations between the cryomilling time, nitrogen concentration, NC-Al grain size, and composite hardness are presented and discussed. The presence of nitrogen as nitride-dispersoids can contribute to the outstanding mechanical properties of trimodal Al MMCs by inhibiting NC-Al grain growth during the high temperature consolidation and deformation process, and by dispersion-strengthening.