Abstract: Since Shihtzupu and Pagoda formations of the middle-upper Ordovician System are named in 1920s, they had been considered to be conformable contact in the Sichuan Basin. In this study, conodont biostratigraphy and carbon isotope stratigraphy are researched in the middle-upper Ordovician in the Liangcun and Huangcao sections around the Sichuan Basin. Both biostratigraphic and chemostratigraphic results indicate a gap between the Shihtzupu and Pagoda formations. The discontinuity can be correlated with the M4/M5 sequence boundary which is a prominent unconformity in North America. Both of them probably result from the regression in the early and late Sandbian. There is also unconformity in the middle-upper Ordovician in Bachu area and Tarim Basin. The possible disconformity between the Shihtzupu Formation and Pagoda Formation is significant for our petroleum exploration in the Sichuan Basin.