AbstractThe western part of the Tarim Basin in Xinjiang is one of the main areas in China where the marine Cretaceous is well developed. The Upper Cretaceous Yingjisha Group represented mainly by sediments of littoral, near‐shore neritic and estuarine facies is divided in ascending order into the Kukebai Formation, the Oytak Formation, the Ygezya Formation and the Tuylouk Formation.For about thirty years, the basal beds of the Kukebai Formation had been considered to be the lowermost marine horizon of the Cretaceous in the western part of the Tarim Basin, which represents the earliest transgression of the Cretaceous Sea into this region. Recently, marine trace fossils,Ophiomorpha nodosa,O. tuberosaandThalassinoides? spp. were found in abundance and fine preservation from the upper subcycle and upper part of the lower subcycle of the Kezlesu Group underlying the Kukebai Formation. The fact indicates that the marine transgression there took place earlier than the Kukebaian.Process of transgression and regression and change of environment in the West Tarim Basin during the period from the late Early Cretaceous to the end of the Cretaceous is also discussed in this pape