The Upper Member of the Middle Triassic Guanling Formation in Luxi, Yunnan yields abundant fossils, including marine reptiles, fishes, arthropods, echinoderms, mollusks, brachiopods and plants and the fossil assemblage in Luxi is similar to that of the Panxian Fauna and Luoping Biota. However, the biostratigraphy study in Luxi is poor, making it difficult to accurately determine its geological age. In this study, LA- ICP- MS U- Pb analysis was performed on volcanic tuff samples from the major fossil sites, including Suomeiluo and Muta sections in Luxi and Gejiehe section in Panxian, Guizhou. Methods: Four volcanic tuff samples were collected from the Suomeiluo and Muta sections in Luxi, Yunnan. One volcanic tuff sample was collected from the Gejiehe section in Panxian, Guizhou. The LA- ICP- MS analysis was carried out in Mineral Laser Microprobe Analysis Laboratory (Milma), China University of Geosciences (Beijing). Before isotopic analyses, the internal structure of the zircons was examined using cathodoluminescence (CL). 30 zircons with clear oscillatory zoning were selected for the analyses in each sample. Uncertainties for each analysis were reported at the 2σ level. Data reduction and U- Pb concordia diagram were carried by Isoplot．Results: The results show that the age of the volcanic tuff sample of the Suomeiluo Section is 243. 9±1. 8 Ma, and ages of three samples of Muta Section from the bottom to top are 245. 2±3. 4 Ma, 243. 7±1. 7 Ma and 242. 2±1. 5 Ma, respectively. The zircon age of the volcanic tuff sample from the Gejiehe Section is 244. 0±1. 5 Ma. Conclusions:According to the dating results, the fossiliferous layers of the Upper Member of Guanling Formation in Luxi are in Pelsonian substage of the Anisian (Middle Triassic), which is chronologically consistent with the Panxian Fauna and Luoping Biota.