The phase formation of NiAl3and Ni2Al3is studied on lateral diffusion couples of an Al‐rich source on a Ni thin film at temperatures from 375 to 500 °C. Analytical electron microscopy is used to determine the crystal structures, chemical compositions, and the widths of growing phases. Simultaneous growth of NiAl3and Ni2Al3is observed at 375 and 450 °C. Al atoms dominate the diffusion process in NiAl3and Ni2Al3. The growth of Ni2Al3has a parabolic dependence on annealing time, and the growth constant,X2/t, has an activation energy of 2.0±0.2 eV. The growth kinetics is further studied by comparing the growth rates of NiAl3in one‐ and two‐phase growth, and by applying the criteria proposed by Go¨sele and Tu to the simultaneous growth of NiAl3and Ni2Al3.