The morphology. the phase structure and the crystallization processes of poly(3-D(-)hydroxybutyrate)/atactic poly( methylmethacrylate) blends. (PHB/aPMMA). have been investigated as a function of blend composition and thermal treatments. The two-phase separation processes (liquid/liquid demixing and liquid/solid separation) are in competition between each other. In the melt at T greater than or equal to 185 C, the system was homogeneous. During crystallization from the melt, liquid/liquid phase separation occurred before crystallization. After crystallization, three phases were present: a crystalline PHB phase; and two amorphous phases, one formed by amorphous PHB and the other by aPMMA. From the homogeneous glass state, however, crystallization preserved the homogeneity. The system. after cold crystallization. was composed of two phases: a crystalline: and an amorphous homogeneous phase. The addition of aPMMA always caused a decrease of the spherulite radial growth and overall crystallization rates. These results are attributed to the increase of energy related to the transport of the macromolecules in the melt, and in the case of cold crystallized samples, also to the diluent effect of aPMMA on the growth of PHB crystals.