A fully bio-sourced and biodegradable composite materials has been developed, based on polyhydroxybutyrate and hemp fibres (20wt% content). Improvements of the final performances of composite materials are not only related to the filler content and intrinsic properties of constituents, but also to the filler dispersion state and to the filler/matrix interface. Hence, the week interactions between the apolar matrix and the polar fibres have been counterbalanced by adding a green compatibilizer. The impact of processing (screw profile and speed) and formulation (compatibilizing route) on the structure and thermal properties of biocomposites has been discussed in relation to their mechanical properties and water vapour sensitivity. A strong nucleating effect provided by the fibres has been highlighted and a clear fibre-matrix load transfer confirmed the effectiveness of the prepared compatibilizer. Biodegradability of those fully biobased composites was also evaluated in compost.