The　dispersion　behaviors　of　multi-walled　carbon　nanotubes　(MWCNTs)　in　water,　using　an　ionic-gemini　type　molecule,　4,　4＇-di　(n-alkyl)　diphenylmethane　disulfate　salt　(C-n-DSDM,　n　=　12,　14,　16)　as　the　dispersant,　were　investigated.　The　MWCNTs　reinforced　magnesium　(Mg)　matrix　composite　was　fabricated　successfully　by　using　the　MWCNTs　dispersed　by　DSDM.　The　effects　of　hydrophobic　chain　length,　sonication　time　and　pH　on　the　dispersibility　of　MWCNTs　were　characterized　by　UV-vis　spectrophotometer,　dynamic　light　scattering,　Raman　spectroscopy,　transmission　electron　microscopy　and　zeta　potential　measurements.　Results　show　that　the　C-n-DSDM　can　effectively　disperse　MWCNTs　in　water　through　a　noncovalent　functionalizing　mechanism,　and　the　dispersion　is　enhanced　with　reducing　in　the　hydrophobic　chain　length,　i.e.　the　C-12-DSDM　shows　the　best　dispersibility　among　the　three　kinds　of　C-n-DSDM.　The　pH　value　of　water　for　the　most　effective　dispersion　is　7,　and　the　dispersibility　is　improved　with　the　increase　in　sonication　time.　The　tensile　test　result　of　the　C-12-DSDM　dispersed　MWCNTs　reinforced　Mg　matrix　composite　demonstrates　that　the　mechanical　properties　especially　the　elongation　are　outstandingly　enhanced,　i.e.　by　addition　of　0.1　wt%　C-12-DSDM　dispersed　MWCNTs,　the　ultimate　tensile　strength　(UTS)　and　elongation　(E)　of　pure　Mg　are　improved　by　about　12.5%　and　50.7%,　respectively.　The　strengthening　mechanism　of　C-12-DSDM-MWCNTs/Mg　composite　was　discussed　in　detail.