Limited biostratigraphic studies have been undertaken on the upper limestone member of the Luolou Formation which is also the only occurrence horizon of the marine reptile in the Olenekian Stage, northwest margin of the Youjiang basin. This article focused on the upper limestone member of the Luolou Formation in the Zhebao and Gannan regions, respectively, east of the Anran anticline. Based on the section and systematic conodont biostratigraphic study, the results show 5 and 3 conodont zones can be identified in the upper limestone member of the Luolou Formation in the Gannan and Zhebao regions, respectively. According to the conodont biostratigraphic correlation of two sections, 5 conodont zones were established in the northwestern margin of the Youjiang basin, Olenekian Stage, and from the bottom upwards 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 Early Triassic marine reptiles in northwest margin of the Youjiang basin mainly belong 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 the Luolou Formation, but also provide evidence for careful comparison of occurrence horizon of the marine reptile fauna, Olenekian Stage.