Our work is focused on the development of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) composite materials loaded with olive husk flour (OHF) to solve the problem of compatibility between the hydrophobic polymer matrix and the hydrophilic fibre. A maleic anhydride (MA) modifying agent was used, which was grafted onto the polymer using a screw extruder. The efficiency of the grafting reaction of maleic anhydride on the macromolecular chains of PHBV with the rates of 0.73% for 1% MA and 0.3% for 3% MA was calculated using a titration method. The results showed that the mechanical properties of the PHBV/OHF bio-composites decreased considerably with increased modulus due to the poor interfacial adhesion between PHBV and OHF. The contribution of the modifying agent was undeniable, cohesion significantly improved and the mechanical properties also increased. The thermal properties showed that the degree of crystallinity of the composites decreases after the addition of a modifying agent. However, in view of the results, better adhesion to the interface is achieved with the use of 1% modifying agent (MA).