The breakage of lignocellulose fibers during melt mixing process have been established in previous studies and applied to predict fiber size changes when compounding composites in a twin-screw extruder. In the present study, the influence of the matrix was characterized, by comparing the results obtained with non-polar polypropylene (PP) and polar poly(butylene succinate) (PBS) of similar viscosity. For flax and hemp fibers, morphology changes (in length and diameter) were characterized along the screws for various processing conditions (screw speed and feed rate). It is shown that, whatever the conditions, breakage is more important with the PBS matrix. Moreover, the rheological behavior of the composites and the mechanical properties in uniaxial extension are also very different, indicating specific interactions between the fibers and the matrices. Measurements of adhesion forces confirmed a better interaction between cellulose and the PBS matrix compared to the PP one.