Different type of fillers (wood flour, hemp, cotton fibres and recycled tetra-pack carton cellulose) were mixed with polypropylene (PP) and polylactic acid (PLA). Obtained composites (filler content 40 wt%) mechanical properties were evaluated. Interfacial adhesion modifiers (maleinated polypropylene), impact strength modifiers (ethylene-octane copolymer) influence on the increase of compatibility and improving of composites properties were discussed. It was shown that the most significant increase of PLA composites flexural strength, modulus and impact strength was observed if as reinforcement recycled tetra-pack cellulose fibers was used.