Poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) biocomposites different sisal, containing with the fiber length of 0.25 and 5 mm, and addition of clay particles were prepared by hot compression technique. Silane (Bis(triethoxysilylpropyl)tetrasulfide) treatment has been used to modify, and thus enhance, the properties of related hybrid composites. All composites were subject to water absorption test. The mechanical properties of hybrid composites, such as tensile stiffness and strength, toughness, and hardness, determined tensile, impact, and hardness tests, respectively. It was found that tensile strength, stiffness, and impact strength of long sisal fiber improved with increasing fiber content. Hardness of short sisal fiber improved with increasing fiber content. Treated Silane of long fibers at 20 wt.% loading was found to enhance the tensile strength fiber by 10% and impact strength by 750% as compared with the neat PHBV. Note that this feature was also confirmed by scanning electron microscopy. Moreover, the hardness and water resistance of the PHBV/sisal composites increased with the addition of clay particles. The diffusion coefficient for the PHBV and hybrid composites systems studied was also calculated. [ABSTRACT FROM PUBLISHER]