We investigated the effect of bulky pendent side groups on the structure of mesophase in a thermotropic main-chain liquid-crystalline polymer, poly[(phenylsulfonyl)-p-phenylene 1,10-decamethylenebis(4-oxybenzoate)] (PSHQ10), synthesized in the present study. For comparison, poly(p-phenylene 1,10-decamethylenebis(4-oxybenzoate)] (PHQ10) having no pendent side groups was also synthesized. Using wide-angle X-ray diffraction, differential scanning calorimetry, and polarized optical microscopy, we made the following observations : (1) the introduction of phenylsulfonyl side groups into the PHQ10 decreased the isotropization temperature by as much as ca. 140 degrees C; (2) PHQ10 undergoes three phase transitions, (i) the solid-smectic transition, (ii) the smectic-nematic transition, and (iii) the nematic-isotropic transition; and (3) PSHQ10 undergoes only two phase transitions, (i) the solid-nematic transition and (ii) the nematic-isotropic transition. The absence of a smectic mesophase in the PSHQ10 is ascribed to the presence of bulky phenylsulfonyl side groups, which hinder the molecular packing, thus exerting a steric effect and depressing the smectic properties. The variable-temperature X-ray diffraction patterns indicate further that at temperatures above the melting temperature of crystals, PHQ10 has an ordered structure which is layered and closely packed while PSHQ10 has a less ordered structure, the difference being attributable to the presence of bulky phenylsulfonyl side groups in the PSHQ10. The combined use of low-angle and wide-angle X-ray diffractions taken at various temperatures enabled us to conclude that the PHQ10 has a high-order smectic mesophase. The polarized optical microscopy results reveal that the PHQ10 has a focal conic fan-textured smectic mesophase at temperatures of about 150-260 degrees C and Schlieren-textured nematic mesophase at temperatures of ca. 260-325 degrees C, while the PSHQ10 has only a Schlieren-textured nematic mesophase at temperatures of ca. 115-175 degrees C during the second heating.