Abstract: The Monte Carlo simulation was carried out for studying the segregation of Mg atom to the Ni₃Al grain boundary. Embedded-atom potentials for Mg-Mg, Ni-Mg and Al-Mg have been obtained based on experimental data and LMTO results. The unrelaxed, starting structure of the grain boundary is 001/∑5（210）/36.87⁰ symmetric tilt boundary according to the coincidence site lattice（CSL）model. The results indicate that the energy of the system when Mg atom occupying the Al site in the bulk is lower than that when Mg occupying the Ni site in accordance with the atomic radius differences between them. The energy when Mg atom occupying the Al site in grain boundary is lower than that when Mg occupying the Al site or Ni site in bulk and at surface so that the total system energy was reduced of about 1 eV if the Mg segregates to the grain boundary.