Abstract: A sequence of Early and Middle Carboniferous strata with hybrid organisms was commonly developed in the Tianshan Mountains region, Xinjiang, after deposition of the Early Carboniferous Datangian strata with large amounts of Gigantoproductus, Striatifera, Kueichouphyllum, Siphonophyllia,Carcinophyllum, Diphyphyllum, etc. and before deposition of the Middle Carboniferous strata with an abundance of Choristites, Profusulinella, Pseudostaffella, etc. The author calls it the transition zone. This zone contains an abundance of tetracorals and brachiopods. The tetracorals consist mostly of the genera and species that continued from the Early Carboniferous, such as Aulina, Yuanophyllum, Lonsdaleia, Lithostrotion, Arachnolasma, Ganganophyllum and Aspidiophyllum, but some common elements of the Middle Carboniferous such as Gshelia, Paralithostrotion and Orygmophllum are also present. Brachiopods consist of some species that have gone through a long geologic course. The common species Choristites of the Middle Carboniferous occurs individually in the trannsition zone, and besides Middle Carboniferous species such as Brachytthylina lata may be recognized. The refore, in the light of the characteristics of the sedimentary formation of this sequence of strata and the general appearance of the biota, particularly the character of tetracorals, the author tentatively assigns this transition zone to the upper part of the Lower Carboniferous.