Poly(hydroxybutyrate-co-hydroxyvalerate)(PHBV) biocomposites containing of sisal fibers and clay particles were prepared by solution casting technique. Silane (Bis(triethoxysilylpropyl)-tetrasulfide) treatment has been used to modify surface of sisal fiber and enhance the properties of related PHBV composites. The mechanical and thermal properties of the PHBV composites were determined in uniaxial tensile, dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC), respectively. The dispersion of the sisal fiber and clay in the PHBV was studied by scanning electron microscopy (SEM). All PHBV based composites were subject to water absorption. It was found that the tensile and storage modulus of PHBV/5 wt.% sisal composites was improved by maximum 7 % and 10 % respectively. Treated silane of sisal fiber at 5 wt.% was found to enhance tensile modulus by 12 % as compared to the neat PHBV. In corporation of 3 wt.% clay in PHBV matrix increased the tensile and storage modulus by 8 % and 16 % respectively, compared to the neat PHBV. Note that this feature was also confirmed by SEM. Moreover, water uptake and the diffusion coefficient of the PHBV composites systems studied was also calculated.