Formation energy of an intrinsic stacking fault in copper is calculated by the use of two interionic potential functions given by Prakash and Lucasson. A modified Ewald method is used to calculate lattice sum of long-range interionic potentials expressed by V p sin cos (2 k F r )/(2 k F r ) 3 . Numerical results show that 1) the two potential functions give 60.5 erg/cm 2 and 78.3 erg/cm 2 to the intrinsic stacking fault energy, and 2) the energy comes mainly from a long-range interaction but not from a short-range one. These potentials are used also for calculations of binding energies of fcc, bcc and hcp structures. Numerical results show that the fcc structure is stable and the binding energies of the bcc and the hcp structures are larger than that of the fcc by 0.06 eV/ion and 0.01 eV/ion, respectively.