The lower Permian jungseong Formation, Taebaegsan basin (Baegunsan Syncline), represents a coal-bearing siliciclastic succession which was later modified by the Songrim or Dabo orogeny. Sandstone perography and clay mineralogy were studied to understand the thermal history of the Jangseong Formation during basin evolution. Petrographic study indicates the Jangseong sandstones are composed of quartz, feldspar, lithic fragments (metamorphic and sedimetary), and varying proportions of matrix and cement. The dominance of quartz(<97%) over feldspar (<1%) and lithic fragments (<2%) classifies most Jangseong sandstones as quartz arenotes or quartzwackes, but lithic graywackes and sublithic arenite locally occur. The diagentic features of these sandstones include mechanical compaction, cemenation by quartz and clay minerals indentified in the Jangseong Formation by X-ray diffraction analysis are late-stage clay pore-filling. Clay minerals isdentified in the Jangseong Formation by X-ray diffaction analysis are illite, kaolinte, and pyrophyllite with a minor amount of chlorite and micas. The illite, kaolinite, and pyrophyllite appear to be largely autjigenic based on their well-crystallinity forms. There authigenic clay minerals form clay minerals form clay coats/rims and late-stage pore-filling cements, Illitecrystallinity shows that the Jangseong formation has been in late-diagenetic zone to early-epizone, which ranges in temperature from $200^{\circ}C$ to $300^{\circ}C$. In assition, kaolinite-pyrophyllite transition suggests that paleotemperature of the formation has reached at least $265^{\circ}C$. Such temperatureis likely to be consistent with homogenixation temperatures of fluid inclusions in quartz veins in the formation. Thus, the Jangseong Formation has been subjected to paleotemperature of about $265^{\circ}C$. The major heat source responsible for paleotemperature may be hydrothermal solutions. The passage of hydrothermal solutions was probably assisted by fractures created during the basin-modifying tectonism of the taebaegsan basin.