Green composites, which are composed of a biodegradable polymer matrix and biomass fillers such as natural fibers or agriculture residues, have drawn a significant interest in academic research and industrial applications. Green composites are designed to be 100% compostable at the end of their life. However, introducing high contents of fillers into the polymeric matrix to make green composites with stiffness-toughness balance has been a huge challenge due to the filler agglomeration and weak filler-polymer interactions. This work combines reactive extrusion and side-feeding technology, to fabricate green composites with a good stiffness-toughness balance while using high biomass contents. Firstly, a super-toughened poly (butylene succinate) (PBS)/ poly (butylene adipate-co-terephthalate) (PBAT) nano-blend was processed in reactive extrusion in the presence of peroxide. The nano-blends were then used as polymer matrix, with 3rd generation biomass – miscanthus fiber and oat hull which were introduced from side-feeder in one-step extrusion processing. The influence of biomass properties such as cellulose and hemi-cellulose contents; biomass dispersion states; and interaction between the polymer and biomass fillers on the mechanical, thermal, rheological and water absorption properties of green composites are evaluated in this research. Acknowledgements The work was done with the support from the Ontario Ministry of Agri-culture, Food and Rural Affairs (OMAFRA)/University of Guelph -Bioeconomy for Industrial Uses Research Program (Project # 030361); the Ontario Research Fund, Research Excellence Program Round-9 (ORF-RE-09) from the the Ontario Ministry of Economic Development, Job Creation and Trade (Project # 053970); and the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Discovery Grants (Project # 400320 and 401111).