The age of the Weimen Group that is exposed in Songpan-Ganzi (Sichuan) fold belt is of great interest. But so far there are no reports of its formation age. During a 1:250000 scale survey of Mianyang area, a nearly 100 m thick tuff bed in the Upper Weimen Group was identified and one sample was collected for high precision SHRIMP dating. A SHRIMP U-Pb weighted mean age of 530.1 ± 4.3 Ma (MSWD = 1.5) was obtained. The zircon Th / U ratios of the sample are from 0.36 to 1.09, and zircon grains show a clear oscillatory banding structure, indicating that zircons are of magmatic origin. The age results suggest that the Weimen Group was formed in the early Cambrian. The new age provides a chronologic basis for understanding the tectonic evolution of the Songpan-Ganzi fold belt.