Biocomposites from polyhydroxybutyrate (PHB) and some bio-fillers such as lignin (L), alpha cellulose (AC) and cellulose nanofibrils (CNFs) were prepared to investigate the effect of the bio-fillers on the properties of PHB by a solvent casting method. The thermal properties by thermogravimetry analysis (TGA–DTG and DTA) and differential scanning calorimetry (DSC) were determined; morphological characterization by scanning electron microscopy (SEM) and structural analysis by X-ray diffraction (XRD), and Fourier transform infraredspectroscopy (FTIR) of the biocomposites were performed. TGA curves showed that the highest values for T 10%, T 50% of the biocomposites were 278.2◦C for PHB+2%AC and 291.7$^{\circ}$ C for PHB+2%CNFs; however, the bestvalue for T 75% was obtained as 381.5$^{\circ}$C for PHB+2%L. According to DTG curves, the best results were found for PHB+0.5%L and PHB+0.5%CNFs. DTA showed an increase in temperature of maximum degradation withloading of lignin and CNFs. The addition of bio-fillers increases Tc and Tm for both first cooling/heating and second cooling/heating. Tc and Tm values for first cooling/healing were found to be lower as compared with second cooling/healing. Furthermore, the addition of bio-fillers acts as a nucleating agent in PHB and SEM pictures showed the porous structure in all biocomposites. SEM images revealed uniform distribution of the reinforcing particles in the polymer at low loadings (0.5wt%), while higher loadings (2 wt%) of L and CNFs contributed to easy aggregation within the PHB matrix. In XRD studies, PHB in the range 5–55$^{\circ}$ shows 6 main peaks. XRD patterns of the PHBbiocomposites revealed 3 main peaks at 13.57$^{\circ}$, 16.87$^{\circ}$ and 22.1$^{\circ}$, and the other peaks disappeared in the patterns.The largest and lowest values of Xc were found for PHB+2%AC and PHB+2%CNFs, respectively.