This work presents selected properties of polypropylene-based (PP) composites modified with an organic filler, namely walnut shells (NUT). Two series of composites were prepared via a twin-screw extrusion process, followed by injection molding. The first series, named PP/NUT, contained 10, 20 and 40 wt% of walnut shell powder, while the second, abbreviated PP/NUT/MAPP, apart from nut filler included polypropylene-grafi-maleic anhydride (MAPP), which was intended to promote adhesion between the polymer and filler. The following measuring techniques were applied: differential scanning calorimetry, thermogravimetry, dynamic-mechanical thermal analysis, and scanning electron microscopy. The measurements showed the filler to have a significant influence on the properties of the PP/NUT composites in comparison with unmodified polypropylene. It was also shown that the addition of polypropylene-graft-maleic anhydride to the formulation enhanced interfacial bonding between the polymeric matrix and filler. Moreover; the introduction of organic filler to the polymeric matrix increased its stiffness without altering the crystallization kinetics of polypropylene.