Biodegradable poly(3-hydroxybutyrate) (PHB) and thermally reduced graphene (TRG) nanocomposites have been prepared successfully via a solution and coagulation method in this work at low TRG loadings. The effects of TRG on the crystal structure, spherulitic morphology, nonisothermal melt crystallization behavior, and isothermal melt crystallization kinetics of PHB in the PHB/TRG nanocomposites were investigated with various techniques. It was found that TRG does not change the crystal structure of PHB but apparently increases the nucleation density of PHB spherulites in the PHB/TRG nanocomposites. Moreover, both the nonisothermal and isothermal melt crystallizations of PHB are enhanced significantly in the nanocomposites because of the efficient nucleating agent effect of TRG. In addition, the isothermal melt crystallization kinetics of neat PHB and the PHB/TRG nanocomposites was analyzed by the Avrami equation. The overall isothermal melt crystallization rates of PHB are increased with deceasing crystallization temperature and increasing the TRG loadings in the nanocomposites; however, the crystallization mechanism remains unchanged in spite of the crystallization temperature and TRG loadings.