Abstract: The Ermaying Formation, previously assigned to Middle Triassic, is distributed in Xiabancheng-Pingquan area and mainly composed of fluvial red sandstone and mudstone rocks. During 1:50000 regional survey, the authors discovered for the first time a layer of rhyo-tuffite in the upper part of the Ermaying Formation. Zircon LA-ICP-MS U-Pb dating of the tuff sample yielded an age of 234.2±2.6Ma(MSWD=3.2). In combination with fossils, intrusive rocks and lithology, the authors hold that a more appropri-ate sedimentary time of the Ermaying Formation in northern Hebei may be Middle to Late Triassic. Furthermore, the appearance of tuff interlayer, as the response to a volcanic events, might be related to the collision between the Siberia Block and the North China Block during Middle to Late Triassic.