Nanophase NiAl powders were synthesized by solid state reaction via ball milling of elemental Ni and Al powders under an argon atmosphere for 0-8 h, 16 h, 24 h, and 48 h. Structural characterization of the nanophase NiAI was performed by x-ray diffraction and transmission electron microscopy. The oxidation behavior of the powders was studied by thermogravimetric analysis. It was found that the Ni + Al = NiAl solid state reaction took place between 4 and 5 hours of mechanical alloying. After 5 h or more of mechanical alloying, the phase of the ball milled particles was the B2 structure, with average grain sizes which decreased with increasing mechanical alloying times. The relationship between the long-range-order parameters and mechanical alloying time was studied. After 5 h, 6 h, 8 h, 16 h, 24 h, and 48 h mechanical alloying, the long-range-order parameter was determined to be approximately 0.82, 0.75, 0.75, 0.75, 0.71, and 0.63, respectively. Iron contamination was observed, resulting from wear of the steel vial and balls. In the mechanically alloyed NiAl nanoparticles, edge dislocations, shear bands, subgrains, distorted regions, and a large number of grain boundaries were observed.