Graphene nanoplatelets (GNP) of various sizes were mixed with polypropylene (PP) in an internal mixer to prepare composites. The effects of mixing conditions, GNP size and concentration in composites were investigated. The composites were characterized at different scales, using electron microscopy, X-ray diffraction, thermogravimetric analysis, and rheometry. It was shown that the PP/GNP composites had to be considered as non-intercalated and non-exfoliated microcomposites. However, the thermal, rheological and mechanical properties of the PP/GNP microcomposites were improved with respect to that of the matrix and similar to those of thermoplastic/organoclay or reduced graphene nanocomposites. In the best cases, storage modulus plateau in the glassy domain was increased by 30% and onset of degradation temperature by 40 degrees C. Finally, in the investigated range of this study, the mixing conditions applied in the internal mixer did not affect the structural and rheological properties of the PP/GNP composites. POLYM. ENG. SCI., 58:1937-1944, 2018. (c) 2017 Society of Plastics Engineers