Abstract: The order-disorder transitions of stoichiometrical AB and A₃B alloys were studied by using effective-atom model based on EAM (embedded atom method) potential.The results give the analytic function form of the free energy in term of the long range order (LRO) parameter,and indicate that order-disorder transition behaviors of stoichiometrical AB and A₃B alloys may be different. This method can be applied to interpret the experimental results of order-disorder transition in stoichiometrical NiAl and Ni₃Al.