The effects of impurities and grafted chemicals on crystallization and thermal degradation behaviors of biosynthesized polyhydroxybutyrate (PHB) are analyzed by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and gel permeation chromatography (GPC). The biodegradation rates of various specimens are also studied. The results of DSC, TGA, and GPC analyses indicate that a small amount of the impurities in PHB could significantly decrease the crystallization rate, crystallinity, and thermal stability of PHB because of a catalytic degradation effect. However, the thermal stability of PHB can be greatly improved by grafting maleic anhydride (MA) onto PHB molecules through melt and mechanical-grafting methods. The crystallization rate and melting temperature of MA-grafted PHB also considerably increase. The melt-grafting method has a higher grafting ratio but its high temperature and shear force environment degrades PHB more seriously during processing than the mechanical-grafting method. The biodegradation rate of PHB is also affected by the impurities and MA grafted. (c) 2008 Elsevier Ltd. All rights reserved.