Nanocomposites of polypropylene (PP)/organically modified montmorillonite (OMM) containing different interlayer organic cations have been prepared by solid-state processing and melt compound without adding compatibilizer. The nanodispersion of OMM in PP is achievable when n-hexadecyl tri-n-butyl phosphonium or octadecyl dimethyl benzyl ammonium is used as an interlayer organic cation. The dispersiveness of OMM in PP improves with increasing the length of shorter carbon chains around positively-charged atoms (e.g. N or P), which coordinates on the surface of montmorillonite, in the organic cation, because the chains block the contact between negatively-charged montmorillonite and nonpolar PP.