The effect of thermal history on the time evolution of the structure of a main-chain thermotropic liquid-crystalline polymer, poly[(phenylsulfonyl)-p-phenylene 1,10;decamethylenebis(4-oxybenzoate)] (PSHQ10), was investigated using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and optical microscopy. For the study, the specimens having the following thermal histories were employed : (a) fully oriented melt-spun fiber, (b) as-cast specimen, and (c) as-cast specimen followed by thermal treatment at 190 degrees C in the isotropic region. We found that when an as-cast specimen was annealed at 130 degrees C, it exhibited an intermediate endothermic peak ata temperature (T-m1) between the crystal melting temperature (T-m2) (similar to 120 degrees C) and the nem8tic-isotropic transition temperature (T-NI) (similar to 180 degrees C) very soon after the annealing began, but when an as-cast specimen was first heated to 190 degrees C in the isotropic region followed by annealing at 130 degrees C, only T-m2 and T-NI appeared until the annealing continued for similar to 40 h and then T-m1 appeared as the annealing continued further. We found, however, that a melt-spun fiber exhibited only T-m2 and T-NI until the annealing continued for similar to 40 h h and then T-m1 appeared as the annealing continued further. By conducting variable heating rate DSC we have concluded that the appearance of T-m1, which represents high-temperature melting crystals, originated from the recrystallization and perfection of the crystals during isothermal annealing; Using WAXD at room temperature and also at elevated temperatures, we confirmed the existence of two forms of crystals in an annealed specimen. During isothermal annealing of the specimens, which-had different thermal histories, on a hot-stage microscope under cross-polarized light at 130 degrees C (which was slightly above T-m2), we made the following observations. (a) When using a fully oriented melt-spun fiber, we observed the; appearance of banded structure initially : and then the formation of high-temperature melting crystals, which made the banded structure disappear as the annealing continued for over similar to 40 h. (b) When using either a melt-spun fiber or an as-cast,specimen, which received thermal treatment at 190 degrees C in the isotropic region, we observed Schlieren textures initially and then the formation of high-temperature melting crystals as the annealing continued for over similar to 40 h. (c) However, when using an as-cast specimen without thermal treatment, we could not observe distinct Schlieren textures because high-temperature melting crystals were formed immediately after the annealing began.