Crystallization kinetics of PP-based composites reinforced with miscanthus stem fragments was studied by DSC and optical microscopy with or without the addition of MA-g-PP. The presence of miscanthus fragments has a clear and noticeable effect onto the crystallization of the PP matrix, with the crystallization peak being shifted from 113°C to 119°C, due to nucleating effect of miscanthus fragments for the crystallization of PP. However, the size of the miscanthus fragments has a negligible effect onto the crystallization of PP. Isothermal crystallization curves at 130°C evidenced that crystallization started earlier for composite materials coupled with MA-g-PP, as also shown by Avrami parameters. The coupling agent does not only affect the nucleation process but also increases the crystallization kinetics. In addition to the crystallization acceleration caused by MA-g-PP, the size of the fragments also affect the crystallization kinetics. However, fragment size only seems to affect the crystallization process if there is a good interaction with PP matrix Seen by optical microscopy, composites coupled with MA-g-PP had a higher number of nucleation sites, started the crystallization earlier and at a faster rate than composites without coupling agent. Crystallization started independently at the surface of the fibers and on the polymer matrix. These results put in light the effect of a coupling agent on the crystallization of a polymer matrix, here polypropylene.