Polymers are used in many practical applications due to their versatility, low cost, easy processing and light weight. However, a drawback of polymers is their poor mechanical performance in structural applications. One way to overcome this limitation is by blending them with reinforcing materials to obtain a composite. Recently, the discovery and development of nanostructured materials as carbon nanotubes and graphene and their use as reinforcing materials in polymeric matrices have allowed the development of new polymeric nanocomposites materials with improved mechanical properties. One inexpensive method to prepare polymeric nanocomposites is the mixing and dispersion of the nanometer size particles in a molten polymeric matrix by using a batch mixer or twin screw extrusion. In this work, polymer nanocomposites of polypropylene with different concentrations of reduced graphene oxide, pure as well as with a functionalizing agent, were prepared and the rheological behavior under shear flow of the molten matrices as well as that of the nanocomposites was evaluated by using capillary rheometry and torsional flow. The changes in the rheological behavior of the nanocomposites as compared to the matrices are presented, and discussed in relation to the graphene contents, its functionalization and dispersion.