Abstract: The lattice parameter variation of stoichiometric Ni₃Al and NiAl during the order-disorder transformation was studied based on the embedded atom method (EAM). The order-disorder transformation for Ni₃Al is of the first order transition, and the lattice parameter changes linearly along with the long range order parameter. NiAl is in the second order transition, and the relationship between the lattice parameter and the long range order parameter is nonlinear. The lattice parameter variation during the order-disorder transformation depends on both the phase transition type and the mechanical equilibrium condition. The lattice parameter should change linearly for the first order transition and nonlinearly for the second order transition.