Polypropylene (PP)-based nanocomposites were prepared using two PPs, namely, PP1 and PP2, which have different flow properties. A low density polyethylene (LDPE) was mixed with the PPs in the composites, and a commercial modified-montmorillonite (15A) was used as a nano-filler. 15A was demonstrated to disperse more finely in the high-viscosity PP1-based matrix than in the low-viscosity PP2-based matrix. The nanocomposites were achieved after the addition of a maleated polyolefin compatibilizer. The 15A retarded the crystallization of both PPs in the composites. Moreover, the PP1-based composites possessed slower crystallization rate compared with the PP2-based counterparts. The melting temperature of the two PPs increased in the composites, wherein PP1 exhibited a slightly higher degree of increase than PP2. Rheological measurements demonstrated that the PP1-based system is more elastic than the PP2-based system. The stiffness of the two PPs decreased after the formation of the composites. The presence of maleated polyolefin compatibilizer caused a significant increase in the impact strength of the composites.