Carbon nanotube (CNT)/magnetite ($Fe_3O_4$) nanocomposites were successfully fabricated by nanocoating. When the CNT/$Fe_3O_4$ nanocomposites were added to a carbonyl (CI) based magnetorheological fluid (MRF), the magnetic rheological properties of the MRF were enhanced. Such enhancement in magnetic rheological properties of the CI-CNT/$Fe_3O_4$ MRF is considered to be attributable to both the superparamagnetic property of the magnetic layer covering the CNT surface and the high length to diameter ratio of the CNT/$Fe_3O_4$ nanocomposite. Impact properties for CI-CNT/$Fe_3O_4$ MRF treated Kevlar fabrics were also tested, and improved impact properties were observed with external magnetic fields compared to CI-MRF treated Kevlar fabrics.