Abstract: The Jilong Sag is a depression within the East China Sea shelf basin. Since Late Cretaceous, the sag has experienced three stages of tectonic evolution, i.e. the rifting, the post-rifting subsidence and the regional subsidence. Thick Cenozoic deposits are formed in the sag. The sag is in fact a half-graben tectonically complicated by faults in the east, but overlapping in the west. The Cenozoic can be divided into three structural belts from west to east namely west fault belt of gentle slope, central belt of depression and eastern belt of steep slope. From Eocene to Miocene there had developed several sets of source-reservoir-cap assemblages, suggesting a high hydrocarbon potential in the Sag.