The phase structure of the miscible blends of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-HV) has been analyzed through DSC measurements and the observations of spherulite growth. Melting temperature and spherulite growth rate of the blends changed depending on the blend composition, which indicates that the extent of phase segregation increases as the HV content of PHB-HV increases from 0 to ca. 30 mol% and as the crystalline temperature increases from 90 to 120 degreesC. When the HV content of PHB-HV is very low, the PHB/PHB-HV blend shows complete cocrystallization. The PHB content in the crystalline phase is similar to the whole composition of the blend. As the HV content of PHB-HV or the crystallization temperature increases, the PHB-HV content in the cocrystalline phase decreases. As a result, a large amount of PHB-HV chains are remained in the miscible amorphous phase between the PHB lamella when the HV content of PHB-HV is over 20% and the PHB-HV content of the blend is large. These PHB-HV chains may form the second and third crystalline phases, which are the PHB-HV crystal and the cocrystals with the PHB chains remaining in the amorphous phase. These data suggest that the cocrystallization of PHB and PHB-HV is induced by the entrapment of the PHB-HV chains near the growing front of the crystals by the PHB chains being about to crystallize.