Natural fillers composed of cellulose, lignin and fillers with a varying lignin cellulose ratio (flax, jute, curaua fibers and miscanthus stem fragments) were used to prepare composites with polypropylene using the same procedure, with or without a maleic anhydride grafted polypropylene (MA-g-PP) coupling agent. A clear acceleration of the crystallization kinetics was observed in the presence of miscanthus stem fragments. For non-coupled composites, the size and aspect ratio of fragments had no significant influence onto the crystallization kinetics of polypropylene. The presence of the MA-g-PP coupling agent increased even more the kinetics. A clear effect of the nature of the polymers present in the filler was observed. There is a direct relationship between the values of the Avrami kinetic constant k for the six fillers and their lignin content, the more effective to enhance crystallization kinetics being pure cellulose. Lignin has no effect onto the crystallization kinetics of polypropylene. (C) 2017 Elsevier Ltd. All rights reserved.