High-resolution transmission electron microscopy was performed on vacuum-deposited nanocrystalline nickel aluminide films. Several nickel aluminide ordered structures, i.e. L1(2)(Ni3Al)-, B2(NiAl)-, D5(13)(Ni2Al3)- and DO20(NiAl3)-type structures, were observed in the deposited films. The L1(2) and B2 ordered structures became unstable with decreasing grain sizes. The critical grain size on transformation from the L1(2)- and B2-type ordered structures into disordered structures was ca. 5 nm at ambient temperatures. High atomic diffusion, sufficient for grain growth, and an increase in the ordering occurred just above 400 degrees C in the nanocrystalline Ni-Al films with L1(2)- and B2-type structures. The diffusion bonding process, at ambient temperatures, between Ni-Al nanocrystallites with an L1(2)-type structure was observed dynamically at atomic resolution under strong electron irradiation. It was found that the nanocrystallites rotated and slid without crack generation, and neck-growth proceeded even at ambient temperatures.