Self-assembly of poly(3-hydroxybutyrate) (PHB) and cellulose acetate butyrate (CAB) blends prepared by solvent casting was investigated by simultaneous synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction as a function of temperature in a heating process of the blends. The results revealed different elemental processes occurring in three characteristic temperature regions: (1) low-T region, where cold crystallization (CC) of PHB into one-dimensional (1D) assemblies of PHB lamellar crystals occurs in the temperature (T) range T-cc,T-PHB <= T <= T-cc,T-CAB, where T-cc,T-PHB and T-cc,T-CAB are the onset temperature for CC in PHB and CAB, respectively; (2) crossover-T region, where partial melting/recrystallintion of PHB occurs concurrently with CC of CAB in the interstitial amorphous matrix among the ID PHB lamellar assemblies at T-cc,T-CAB < T <= T*, where T* is a characteristic temperature close to the T at the higher melting endothermic peak of differential scanning calorimetry (DSC), T-m,T-DSC; and (3) high-T region, where PHB crystals melt and CAB partially melts/recrystallizes at T > T*. The study also elucidated for the first time in this blend systems, to our best knowledge, the following points: (a) the existence of two glass transition temperatures (T-g's) in the amorphous matrix, with a lower T-g (T-gl) determined by DSC and a higher T-g (T-g,T-h) determined from the SAXS intensity at low q (magnitude of the scattering vector) values as a function of T; (b) the evolution of the large length-scale composition fluctuations in the amorphous matrix of the blend at T-g,T-h <= T <= T-cc,T-CAB; (c) crystallization of CAB at T > T-cc,T-CAB and a melting temperature for CAB crystals higher than 183 degrees C. The roles of intermolecular hydrogen bondings (HBs) between PHB and CAB, intramolecular HBs within PHB, and those within CAB as well as roles of the exchanges of those HBs on the self-assembling process, the composition fluctuations, and the miscibility of the blends in the amorphous phase were also clarified in the text.