To improve the mechanical properties of biode-gradable bioplastic poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), a filler made up of silica particles (SiO2) is mixed with PHBH; however, SiO2 shows low compatibility with PHBH. Herein, to enhance the interfacial adhesion of SiO2 and PHBH, the PHBH-grafted SiO2 (SiO2-PHBH) fillers were prepared from propargyl-terminated PHBH and azide-modified SiO2 by click chemistry. Scanning electron microscopy revealed that SiO2-PHBH fillers dispersed uniformly in PHBH. In contrast, the SiO2 fillers aggregated in PHBH. Thermal analysis of the PHBH/SiO2 -PHBH composite films revealed that the glass transition temperature shifted to a high temperature. Young's modulus and yield stress of the PHBH/SiO2-PHBH composite films were increased with increasing concentration of SiO2-PHBH. Conversely, PHBH/SiO2 composite films showed only a small increase in the elastic modulus and a decrease in the yield stress. These results confirmed that SiO2-PHBH improves the mechanical properties of PHBH, and it is expected to expand the industrial applications of PHBH.