Behaviors of C or O in bcc Fe and interactions of C-O and oxygen-carbon-vacancy (O-C-□) are investigated by first principles calculations.Octahedral interstitial site is the most stable position for an O atom in bcc Fe.The migration energy of an O atom in bcc Fe is 0.46 eV.The strength of O-Fe (1nn) bond (0.32) is slightly greater than that of Fe-Fe metallic bond (0.26).Repulsive interactions of C-C,O-O,and C-O exist in bcc Fe.When the concentration of FIA (FIA refers to C or O) is relatively high,a vacancy can attract four FIAs and form stable FIAs-□ complex.