Ni x Al 1 0 0 - x (x=70, 75, 80at.%) alloys were obtained by planar flow casting. The phase selection behavior of the alloys under different processing conditions was studied by using X-ray diffraction and electron microscopy. In contrast to the conventional casting ingots, the volume fraction of γ'-Ni 3 Al in the melt-spun ribbons increased and the volume fraction of β-NiAl decreased. In the alloys Ni 7 0 Al 3 0 and Ni 7 5 Al 2 5 , this trend is mainly due to the domination of the γ' phase in the competition with the β phase, caused by the higher kinetic undercooling of the γ' phase during rapid solidification. The effects of melt quenching on the phase compositions were explained on the basis of the kinetic analyses of nucleation and dendrite growth of coexisting intermetallics γ'(L1 2 ) and β (B2) phases.