The thermal behavior and molecular interaction of a new type of bacterial copolyester, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), P(HB-co-HHx) (HHx = 12 mol %), was investigated by using wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The WAXD measurements were carried out over a temperature range from 25 to 110 degreesC in the scattering angle range of 20 = 5-13degrees. The WAXD pattern at room temperature shows that the P(HB-co-HHX) copolymer has an orthorhombic system (alpha = beta = gamma = 90degrees) with a = 5.76 Angstrom, b = 13.20 Angstrom, and c = 5.96 Angstrom (fiber repeat), which is identical to the crystal system of poly(3-hydroxybutyrate) (PHB) homopolymer. However, temperature-dependent variations in the lattice parameters, a and b, of P(HB-co-HHx) are quite different from those of PHB. Only the a lattice parameter increases with temperature, while the b lattice parameter changes very little in the case of crystalline P(HB-co-HHx). It seems that the intermolecular and intramolecular interactions between the C=O group and the CH3 group decrease along the a axis of crystalline P(HB-co-HHx) with temperature. The (110) peak area of P(HB-co-HHx) starts decreasing from around 50 degreesC while that of PHB changes little at least until 140 degreesC, indicating that the crystallinity of PHB remains almost unchanged until 140 degreesC, but that of P(HB-co-HHx) decreases gradually from fairly low temperature (similar to50 degreesC). The DSC measurement of the chloroform solution cast P(HB-co-HHx) shows a recrystallization peak around 51 degreesC. The (110) peak area of WAXD pattern of the chloroform solution cast P(HB-co-HHx) shows a maximum value at around 54 degreesC. The maximum of the (110) peak area demonstrates the recrystallization process of P(HB-co-HHx). The result is in a good agreement with the result of the DSC measurement.