Using “tunable” interatomic potentials, the lattice misfits for a fcc–bcc metal system have been varied in atomistic models, while keeping other properties essentially unchanged. The procedure and the fitting results of such tunable interatomic potentials for fcc–bcc systems are presented. Varying lattice misfits were found to significantly alter the atomic structure of fcc–bcc interfaces in Kurdjumov–Sachs crystallographic orientation. Defect formation energies at the interfaces were calculated. For vacancies, in general, high numbers of low energy sites are associated with high dislocation junction densities. For interstitials, the formation energies are all substantially below the bulk value, regardless of lattice misfits. These results are relevant to understanding the sink strength of interfaces with different atomic structures.