The miscibility and phase structure of binary blends of poly[(S)-lactide] (PLA) (M-w = 680, 000) with atactic poly[(R,S)-3-hydroxybutyrate] (ataPHB) of different molecular weights (M-w = 9400, 21,000 and 140,000) have been investigated by the means of differential scanning calorimetry (DSC) and optical microscopy. DSC thermograms for the blends of PLA and ataPHB with M-w = 9400 in the range from 0 to 50 wt.% of ataPHB content showed single glass transition temperature after melting at 200 degrees C, and the value decreased from 59 to 10 degrees C with an increase in the ataPHB content, indicating that the PLA and low molecular weight ataPHB (M-w = 9400) are miscible in the melt at 200 degrees C within the ataPHB content up to 50 wt.%. In contrast, the binary blends of PLA with high molecular weight ataPHB (M-w = 140, 000) showed two glass transition temperatures indicating that the binary blend is immiscible in the melt. The radial growth rate of PLA spherulites were accelerated by the addition of low molecular weight ataPHB components. The solid-state structures of melt-crystallized films for miscible blend of PLA with low molecular weight ataPHB were characterized by DSC, wide-angle X-ray diffraction, small-angle X-ray scattering and dynamic mechanical thermal analysis. X-ray crystallinities of PLA components in the Melt-crystallized films were increased by the addition of small amount of ataPHB component, suggesting that the addition of small amount of ataPHB-3 component facilitated the crystallization of PLA components in the binary blends. The lamellar thickness of PLA crystals decreased slightly with an increase in ataPHB content, suggesting that ataPHB component was incorporated into the interlamellar region of PLA spherulites. The relaxation phenomena were detected at two different temperatures in the dynamic mechanical spectra for PLA/ataPHB blends (containing ataPHB contents over 15 wt.%) crystallized at 120 degrees C, indicating that the partial phase separation of two components occurs in the amorphous phase during the isothermal crystallization process.