Abstract: A modified embedded-atom method (MEAM) interatomic potential for the Fe−Cu binary system has been developed using previously developed MEAM potentials of Fe and Cu. The Fe−Cu potential was determined by fitting to data on the mixing enthalpy and the composition dependencies of the lattice parameters in terminal solid solutions. The potential gives a value of 0.65 eV for the dilute heat of solution and reproduces the increase of lattice parameter of Fe with addition of Cu in good agreement with experiments. The potential was used to investigate the primary irradiation defect formation in pure Fe and Fe−0.5 at.% Cu alloy by a molecular dynamics cascade simulation study with a PKA energy of 2 keV at 573 K. A tendency for self-interstitial atom-Cu binding, the formation of mixed (Fe−Cu) dumbbells and even Cu−Cu dumbbells was observed. Given a positive binding energy between Cu atoms and self-interstitials, Cu transport by an interstitial diffusion mechanism could be proposed to contribute to the formation of Cu-rich precipitates and irradiation-induced embrittlement in nuclear structural steels.