Electrical properties of single-walled carbon nanotubes (SWCNTs) filled with Fe are studied by fabricating them as the channels of field-effect transistor devices. The synthesis of Fe-filled SWCNTs is realized by using ferrocene as the starting material. Our results reveal that ferrocene-filled SWCNTs show the interesting ambipolar behavior. In contrast, Fe-filled SWCNTs can exhibit high performance unipolar n-type semiconducting characteristics, suggesting the possibility of creating ferromagnetic semiconducting SWCNTs. Moreover, Coulomb blockade oscillations are significantly observed on Fe-filled SWCNTs, which indicates that they exhibit excellent single-electron transistor characteristics at low temperatures.