Abstract: P(3HB-co-4HB)-PLA blend and glass fiber(GF) treated by hydrochloric acid or coupling agent were selected as the matrix and the reinforced material respectively to prepare GF/P(3HB-co-4HB)-PLA composites via melt blending in order to improve its mechanical properties and dimensional stability and enlarge its application fields. Fourier transform infrared spectroscopy(FTIR), scanning electron microscope(SEM), thermo gravimetric analyzer(TGA) and universal testing instrument etc. were used to investigate the influence of the surface treatment methods of GF on the mechanical and thermal properties, dimensional stability and fracture surface of the composites. The results show that the comprehensive properties of P(3HB-co-4HB)-PLA blends have been enhanced obviously by adding the surface-modified GF. The GF surface grafted with coupling agent can be homodispersed in the P(3HB-co-4HB)-PLA matrix to obtain better interfacial bonding. The tensile strength, flexural strength, notched impact strength and hardness of the composites with 20%(mass fraction) of GF modified by coupling agent increase by 29.38%, 20.32%, 41.38% and 15.31% respectively. The initial decomposition temperature(IDT) and vicat softening temperature(VST) of the composites increase by 6.64 ℃ and 10.7 ℃ respectively. The dimensional stability of the composites in the length direction increase by 32.47% and 33.70% respectively at room temperature or 60 ℃ for 60 d.