Based on research results of lithological, stratigraphical characteristics, tectonic movements, the author clarifies the size and geological structure of the Cöu Long Basin. On the geological side the Cöu Long Basin includes stratigraphic formations and corresponding hydrogeological formations. It is formed and developed in favourable conditions for the formation and preserve groundwater aquifer (and hydrocarbon bearing formations as well). Through hydrodynamic and hydrochemical properties of groundwater, combined with geological conditions mentioned above, the paper demonstrates that the Cöu Long Basin is an artesian basin.