Abstract: The Guanyinqiao Formation, lying between the Glyptograptus persculptus zone at the base of the Lower Silurian Longmaxi Formation and the Paraort- hograptus-Diceratograptus mirus at the top of the Upper Ordovician Wufeng Formation, yields an abundance of fossils of the Hirnantia fauna. It repre- sents the range of the Diplograptus bohemicus zone. The formation confor- mably overlies the Longmaxi Formation and underlies the Wufeng Formation. Samples for dating were collected from the Quanyinqiao Formation and the base of the Longmaxi Formation, the sampling thickness being 1.1 m. The whole-rock Rb-Sr isochron dating indicates that the data points fit in well with a straight 1ine whose ⁸⁷Sr/⁸⁶Sr initial value is 0. 7095±0. 0005 and isochron age 439±18Ma （95% creditability）. This age va1ue agrees well with that for the Silurian-Ordovician boundary commonly used by geologists in the world. Problems about the false isochron line and the duality mixing system are also discussed in this paper.