First-principles electronic structure calculations have been performed for defect and magnetic structures in FeCo. The compositional dependence curves both of formation energies and lattice parameters are obtained by calculations employing supercells of various sizes. The vacancy formation energies are calculated with taking into account the compositional dependence of chemical potentials. Antisite atoms compensate the deviation from the stoichiometric composition both in Fe-rich and in Co-rich FeCo. The compositional dependence of magnetic moments is well explained by the electronic structures of antisite atoms. The vacancy formation energies obtained in the present work show a better agreement with the experimental value than those previously reported.