Abstract:The upper limestone member of Luolou Formation is in low degree of biostratigraphic study, also the only occurrence horizon of the marine reptile, Olenekian Stage, northwest margin of Youjiang Basin. This article focused on the upper limestone member of Luolou Formation in Zhebao and Gannan region, respectively, eastern of the Anran anticline, based on the section and systematic conodont biostratigraphic study, the results show the Olenekian Stage contains 5 conodont zones from the bottom up as follows: The Neospathodus pakistanensis zone, the Novispathodus waageni zone, the Neospathodus crassatus zone, the Icriospathodus collinsoni zone and the Triassospathodus homeri-Tr. symmetricus assemblage zone, the marine reptiles in northwest margin of Youjiang Basin mainly belongs to the Triassospathodus homeri-Tr. symmetricus assemblage zone. The results of this study not only provide reliable biological basis for age limit of the reptile fauna in the upper limestone member of Luolou Formation, but also provide evidence for careful comparison of occurrence horizon of the marine reptile fauna, Olenekian Stage.