The fractionated crystallization and self-nucleation behavior of poly(ethylene oxide) (PEO) component in the miscible PEO/poly(3-hydroxybutyrate) (PHB) binary blends were investigated using differential scanning calorimetry (DSC) and small-angle X-ray diffraction (SAXS) under different crystallization conditions. The distribution of PEO component in the PEO/PHB blend greatly influences its fractionated crystallization behavior. The active heterogeneities, on which the semicrystalline components in the molten state can nucleate at a small supercooling, are favored to locate out of the interlamellar regions of PHB crystals during the crystallization of PHB component. The PEO component confined in the interlamellar regions of PHB crystals crystallizes at an extremely large supercooling, which is induced by the homogeneous nucleation or less active heterogeneities, while the PEO component expelled out of the interlamellar regions of PHB crystals can crystallize at a small supercooling due to the nucleation induced by active heterogeneities. In addition, the self-nucleation behavior of PEO component in the PEO/PHB blend is also affected by the active heterogeneities and the distribution of PEO phase in the blend. Different from the block copolymer systems, the PEO component confined into the interlamellar regions of PHB crystals can not be self-nucleated in the PEO/PHB blend without a nucleating agent (NA). (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 3013-3022, 2010