Abstract: The Yily Basis in the central Tianshan orogenic belt accommodates a huge thickness (about 5000-9000m) of continental Permian strata. The Upper Permian is chiefly composed of fluvial-lacustrine deposits bearing fossils of sporo-pollen, bivalves, ostracodes, and plants. The Xiaoshansayi Formation discordantly overlying the Wulang Group of Lower Permian consists of 419m thick lacustrine mudstone, sandstone and conglomerate with the Cordaitina-Hamiapollenites-Vittatina sporo-pollen assemblage. The Hamist Formation comprises volcanic rocks and coarse clastic sedimentary rocks yielding bivalve Palaeonodonta, Anthraconauta. Argillaceous rocks of lacustrine facies dominate the Tiemulike and the Bakaluesayi Formation, but are frequently interbedded with massive conglomerate beds in the latter. “MPPAO” bivalve fauna and a similar sporo-pollen assemblage occur in these two formations. The Basrgan Formation of the uppermost Permian is composed of fluvial-alluvial conglomerate and sandstone, and reaches 650m thick. The fossil assemblages, lithological features and sedimentary facies of Upper Permian in the Yily Basin show a strong similarity to those of the Junggar Basin.