This study aimed to prepare poly (3-hydroxybutyrate-co-3-hydroxyvalerate), biocomposites with incorporating various percentages of calcium carbonate using extrusion processing. Calcium carbonate was synthesized in the absence and presence of poly(vinyl sulfonic acid). The polymorph and morphology of calcium carbonate chanced with the introduction of poly(vinyl sulfonic acid). The rhombohedral calcite was obtained in the absence of poly(vinyl sulfonic acid). Rhombohedral calcite transformed into spherical vaterite with the addition of poly(vinyl sulfonic acid). The influence of filler contents on the properties of poly (3-hydroxybutyrate-co-3-hydroxyvalerate) composites was studied. The structure and properties of poly (3-hydroxybutyrate-co-3-hydroxyvalerate)/ calcium carbonate biocomposites were investigated by XRD, FTIR, TGA, DSC, SEM, OTR and DMA. The nucleation effect of the calcium carbonate on the crystallization of poly (3-hydroxybutyrate-co-3-hydroxyvalerate) was observed in the DSC and XRD measurements by increasing crystallinity of poly (3-hydroxybutyrate-co-3-hydroxyvalerate). It was shown that the variation of the barrier properties of biocomposites was influenced by polymorph and morphology of calcium carbonate. The addition of 0.5 wt% of the rhombohedral calcite and spherical vaterite increased the barrier properties by 25% and 12%, respectively compared to neat polymer. The dynamic mechanical properties of composites based on rhombohedral calcite and spherical vaterite in poly (3-hydroxybutyrate-co-3-hydroxyvalerate) matrix were investigated. The storage modulus increases by adding both particles in the composites over a wide range of temperature (-30 to 150 degrees C) where the reinforcing effect of calcite and vaterite was confirmed. At the same loading level, rhombohedral calcite led to more increase in the storage modulus, while less increase in storage modulus was observed in the presence of spherical vaterite particles.