Main chain thermotropic liquid-crystalline poly[(phenylsulfonyl)-p-phenylene 1,10-decamethylenebis(4-oxybenzoate)] (PSHQ10) samples having the following thermal histories: (a) fully oriented melt-spun fiber, (b) as-cast specimen, and (c) as-cast specimen followed by thermal treatment at 190°C in the isotropic region were investigated by DSC, WAXD and optical microscopy. We found that when an as-cast specimen was annealed at 130°C, it exhibited an intermediate endothermic peak at a temperature (T m1 ) between the crystal melting temperature (T m2 ) (∼ 120°C) and the nematic-isotropic transition temperature (T NI ) (∼ 180°C) very soon after the annealing began, but when an as-cast specimen was first heated to 190°C in the isotropic region followed by annealing at 130°C, only T m2 and T NI appeared until the annealing continued for ∼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 ∼ 40 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 °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 ∼40 h. (b) When using either a melt-spun fiber or an as-cast specimen, which received thermal treatment at 190°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 ∼40 h. (c) However, when using an as-cast specimen without thermal treatment, we could not observe distinct Schlieren mixtures because high-temperature melting crystals were formed immediately after the annealing began