The results of positron lifetime and binding energy calculations for relaxed vacancy-tyqe defects as well as for grain boundaries in Ni3Al are presented. For this purpose, the so-called atomic superposition method and linear-muffin-tin-orbital techniques are employed. The atomic configurations of defects considered are obtained using molecular dynamics based on an N-body empirical potential. The effect of atomic relaxations around defects on positron characteristics is discussed. Finally, positron lifetime experiments on an electron-irradiated polycrystalline Ni76Al24 Sample. are performed. The positron lifetimes extracted from these measurements are compared with theoretical calculations.