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Density functional theory (DFT) calculations were performed by using FP-LAPW method to investigate the effect of defects on the electronic and magnetic structure of Fe2MnSi Heusler alloy. The formation energies were calculated for antisite, swap, and vacancy defects. We found that three defects, namely MnFe and MnSi antisite as well as FeMn swap defects may spontaneously occur as they have negative formation energies. Whereas, Mn and Si vacancies, FeSi swap, and FeMn antisite defects are unlikely to occur. However, the rest of the defects (FeSi, MnSi, and SiFe) antisites, MnSi swap, and Fe vacancy defects have relatively low formation energies that suggest higher probabilities to occur. The band gaps of the defected structures are found to be smaller than the ideal structure with almost zero values in the cases of FeSi antisite and FeSi swap defects. The structure with Fe vacancy defect, however, exhibits almost the same energy gap as that of the ideal structure. The magnetic structure was also affected by the presence of defects except for FeMn and MnSi swap defects that show the same total magnetic moment of 3 mu B/f.u. as the ideal structure. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim