The paper is focusses on the study of the thermal and mechanical behaviour of reinforced agro-materials. Various formulations based on plasticized starch matrix have been carried out, varying matrix formulation, filler content, fibres length and nature. Cellulose and lignocellulose fibres, which show unequal surface tensions, have been tested. After extrusion and injection moulding, the properties of these wheat starch-based biocomposites are analysed. Mechanical properties (tensile tests), thermo-mechanical properties (DMTA) and thermal degradation (TGA) are analysed. DMTA analysis shows important variations of main relaxation temperature, which can be linked both, to interactions resulting in a decrease of starch chain mobility and to a regular reinforcing effect. These results are consistent with the static mechanical behaviour, which vary according to the filler content (up to 30wt%), fibre nature (cellulose vs. lignocellulose) and fibre length (from 60 μm to 1 mm). Besides, we have shown that the addition of cellulose fillers improves the thermal resistance of these biocomposites. Finally, we have tested the impact of the addition of biodegradable polyesters into these composites without significant effect on the post-processing stability. © 2004 Elsevier Ltd. All rights reserved.