A nanocomposite of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH)/ montmorillonite (MMT) modified with silane coupling agent 3-aminopropyltriethoxysilane (KH550) (PHBH/MMT/KH550) was prepared by solution casting method. The effects of KH550 content on properties of nanocomposites were comparatively investigated. According to the Fourier transform infrared spectra, the new chemical bonding between the PHBH and the MMT was formed by adding KH550. The crystallinity, thermal properties, mechanical properties, and barrier properties of the nanocomposites were improved because of the good dispersion of MMT. Compared with pure PHBH, a 220.05%, 158.25%, and 58.91% improvement in young's modulus, tensile strength, and elongation at break was obtained. The water vapor transmission coefficient was decreased by 113.30%. Moreover, it was notable that the nanocomposite possessed the optimal mechanical properties when the KH550 content was 5 wt%.The obtained results revealed that the nanocomposite will expand the practical application of PHBH base plastics as a substitute of traditional petrochemical materials.