Abstract: The Xibiehe section, as the original standard of the Xibiehe Formation, is located at the Bateaobao area in Darhan Mumingan Joint Banner, Inner Mongolia.Its age has been much debated. The conodont study clearly shows that the lower and middle part of the Xibiehe section should be assigned to the Late Silurian Xibiehe Formation(Ludlow to Pridoli) based on the finding of Ozarkodina uncrispa which can correlate with the lower part of Ozarkodina crispa Zone.The upper part of this section called the Alugong Formation, belongs to the Lochkovian of early Devonian, because typical Lochkovian conodont genus Ancyrodelloides and Lanea have been found. The Xibiehe section is not suitable for being the standard section of the Xibiehe Formation, however, the Baoerhantu section can be used as a standard section of the Xibiehe Formation.

Key words: conodont, Xibiehe Formation, Alugong Formation, Silurian, Early Devonian, Inner Mongolia