The annealing and melting behavior of poly[(R)3-hydroxybutyrate] (P(3HB)) single crystals were followed in real time by synchrotron small- (SAXS) and wide-angle X-ray scattering (WAXS) measurements. The real-time SAXS measurements revealed that the P(3HB) single crystal exhibits a discontinuous increase of lamellar thickness during heating. The structural changes as observed by SAXS and WAXS were in response to the thermal properties of single crystals characterized by differential scanning calorimetry.