Abstract: The Balonggongge'er Formation is widely distributed in the South Qilian Mountain. Due to the absence of the fossils and available geochronologic data, its geological epoch has aroused much attention. Based on 1:50000 regional geological survey, the authors identified some volcanic flows, which consist of dacites within the Balonggongge'er Formation. LA-ICP-MS zircon U-Pb analyses indicate that a dacite sample was formed at 457.6±2.4Ma, belonging to the Late Ordovician period. This is the first precise radiometric age for the Balonggongge'er Formation, which provides some persuasive geochronologic evidence for the disintegration and the epoch of the formation. The new discoveries and results are of importance to the Early Paleozoic stratigraphic sequence and tectonic evolution in the South Qilian Mountain.