In this work, the graft polymerization of methyl methacrylate (MMA) monomer onto short leather fibers (SLF) was investigated as a function of the monomer/leather fiber ratio. This chemical modification was made by aqueous emulsion polymerization initiated by a redox system. The effect of the monomer concentration on the grafting parameters (deposited and grafted polymer, as well as grafting efficiency) were determined. Composites formulated with SLF without chemical modification have showed lower tensile and impact properties in comparison with composites formulated with treated fibers. However, the elongation at break values for both systems remained similar as the MMA content changed. The morphology of SLF grafted with MMA was examined by both optical light microscopy (OLM) and scanning electron microscopy (SEM). Micrographs have shown polymer deposition on individual fibers and bundles of SLF. They have also revealed that PMMA may interpenetrate the SLF network and be deposited in large and coarse aggregates around individual fibers, but without occupation of the free space in the fiber net.