By using atomic-force and scanning electron microscopies (AFM, SEM), wide-angle and small-angle X-ray diffraction (WAXD and SAXS), and thermal and infrared spectroscopy characterization, mechanisms and correlations between lamellar reassembly, phase separation domains, and birefringence patterns in crystallized spherulites in poly(hydroxyl butyrate) (PHB) were probed in detail. Crystallization-induced phase separation in PHB blended with amorphous poly(methyl acrylate) (PMA) produces pebble-like PMA domains, which influence the birefringence types (positive vs negative types) of spherulites in crystallized blends with respect to composition and temperature. Extents of crystallization-induced phase separation were dependent on temperature and composition; for a fixed PHB/PMA (60/40) blend composition the phase separation extent was inversely proportional to temperature of crystallization (T-c). Correspondingly, depending on the extents of phase separation (counted as quantitative ratio of pebble-like PMA domains divided by entire area of the blend samples), the Maltese cross in spherulite patterns of PHB in the blend may rotate from the original positive-type spherulites at T-c = 90 degrees C to negative-type spherulites at T-c = 60 degrees C. The SAXS analyses showed the thickness variation of PHB lamellae (or PHB + amorphous polymer long periods) with T-c was not an influencing factor, but that the formation of grains in the PMA domains from the crystallization-induced phase separation could majorly influence the optical birefringence. Correlations among the phase domains, lamellae assembly, transitions from positive-type, unusual-type, and negative-types of crystallized spherulites in polymers have been expounded upon.