Finally, the studies carried out allowed the influence of filler particle size on the properties of the composites to be determined. In general, the nanoparticles yielded better mechanical properties for both matrices, including tensile strength, elongation at break and impact strength, as well as greater melt stability in rheological tests, due to their better, albeit incomplete, dispersion in the polymer matrix and, consequently, the larger surface area for interaction with the polymer matrix. The studies also showed that in the case of PCL, HAp nanoparticles had a greater affinity for the matrix compared to microparticles.