Abstract: Using laser ablation multiple collector inductively coupled plasma mass spectrometry （LA-ICP-MS）method, the authors determined U-Pb isotopes of the zircon from the dacitic tuffite interlayer of Bayan Har Group pyroclastic rock system of Yazhakangsai area in west Kabaniuerdo Lake of northern Tibetan Plateau. Except for one inherited zircon whose 206Pb/238U age is 419±3Ma, the 206Pb/238U ages of 20 magmatic zircons vary from 242Ma to 254Ma, with the average 206Pb/238U age calculated from 7 samples being 244±2Ma（MSWD=1.3）, which suggests that the formation of Bayanhar Group in this region took place in early Middle Triassic rather than in Late Triassic as considered by previous researchers. The determination of the stratigraphic age provides new evidence for further study of composition and formation age of Bayan Har Group.