Thermal annealing behavior and structure development of crystalline hard segment (HS) (43 wt%) domains in a melt-quenched multi-block thermoplastic polyurethane (TPU) have been investigated using differential scanning calorimetry, atomic force microscopy, wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). A single endothermic peak was observed by annealing the melt-quenched TPU at wide temperature (T-a) region between 25 and 150 degrees C, while it was not observed in the melt-quenched TPU. Interestingly, the onset temperature of the endothermic peak linearly increased with a slope of unity against T-a and was observed at about 14 degrees C above T-a. The WAXD peak of the melt-quenched TPU was sharpened and increased by annealing. The quantitative SAXS analysis by fits using the ellipsoidal form factor with the Percus-Yevick equation revealed that the crystalline HS domain largely grew in the preferential direction along the chain axis by annealing the melt-quenched TPU. These results suggest that the endothermic peak is attributed to the HS domain consisting of the fringed micelle-like crystal. On the other hand, the number density of the HS domain decreased with increasing T-a. This growth behavior of the HS domain might be induced from the coarsening driven by phase separation between HS and SS, in which the smaller HS domain disappeared and redeposited onto the larger HS domain. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.