Abstract: The Borehole CSDP-2 of the Continental Shelf Drilling Program (CSDP) is the only deep hole ever drilled on the central uplift of the South Yellow Sea (SYS). As a reference well, it is critical important to the dating of marine Paleo-Mesozoic deposits and reconstruction of the paleo-environmental pattern and paleo-tectonic settings of the SYS region. Ammonites are recognized as excellent index fossils for approaching the above objectives. So far, many ammonite fossils have been found in the core of CSDP-2, of which two, namely Roadoceras cf. sinense and Ophiceratidae, are specifically significant. The fossil of Roadoceras cf. sinense was discovered at the depth of 1475.2 meters. It is an index fossil distributed from Late Roadian to Wuchiapingian temporally and in the palaeoequatorial and bi-temperate regions spatially. The new finding of Roadocerascf. sinense from CSDP-2 extended the distribution area of this genus to the further north. On the other hand, Ophiceratidae, which is found at a depth of 851.9 meters and belongs to Griesbachian, is very close to the bottom of Triassic, and valuable for study of Permian and Triassic boundary. The discovery of the ammonites is important for confirming the stratigraphic age of the Permian and Triassic marine formations on the central uplift of the SYS region, recovering the paleo - environmental pattern, and providing the foundation for the future study of sedimentation, tectonic and petroleum resources.