New PHBV/wheat straw fibres biocomposites were produced in such a way to improve the fibre/matrix interface and modulate functional properties of materials. A torrefaction treatment was applied to wheat straw fibres in such a way to increase their hydrophobicity. This improvement led to a better fibre/matrix interfacial adhesion in resultant composites prepared by melt-extrusion, as revealed by SEM observations. The crystallization process was favoured by the presence of torrefied fibres while molecular weight and crystallinity of matrix polymers remained unchanged as compared to untreated fibres. Fibre torrefaction treatment had no significant effect on the mechanical properties of PHBV/wheat straw fibre composites, except for 30 wt.% of torrefied fibres with materials displaying an increased rigidity. It was concluded that improving fibre/matrix adhesion and increasing fibre aspect ratio did not lead to a better preservation of mechanical properties, due to the inevitable presence of microscopic defects in the composite materials. As regards water vapour permeability (WVP), a 30% decrease was first noticed with torrefied fibre contents up to 20 wt.%, which was ascribed to the hydrophobic nature of fibres and improvement of fibre/matrix adhesion. WVP then increased for a fibre content of 30 wt.%, probably due to the creation of percolating pathway.