The Tarn Giang-Cau Hai lagoon is the most important coastal lagoon of Vietnam located in Thua Thien-Hue province. Basically formed in the late Holocene (more than 2000 years ago), the lagoon is being in the development stage. Its tidal inlets, nowadays are the Thuan An and Tu Hien inlets, are dynamic and ephemeral morphological features. Inlet migration and shoal, breakthrough ofthe sand barrier, erosion ofbeaches and sand dunes affect on socio-economic development and environment of the province to a high degree. Serious consequences of these processes are adverse effects on flooding and inundation, transportation, navigation, fishery, aquaculture, agriculture, lagoon ecosystem and environment. As a primarily step of research on the system, the study is limited on the hydraulic characteristics of the system with the main objectives are to set-up a numerical model to simulate and investigate the hydraulic behaviour of the system; to evaluate the stability situations of the inlets; and to suggest which processes and data are relevant for the successive steps ofthe study on morphology of the system. DUFLOW has been employed to simulate the hydraulic behaviour of the system under different boundary conditions of sea water level, river flow discharge, inlet geometry and configuration. Sensitivities and effect of the uncertainty of sea level rise, storm surges, inlet openings, river flows and tidal parameters on the hydraulic characteristics of the system have been also investigated. Stability ofthe inlets has been evaluated accordingly. Model results indicate that river flows are the most important acting force of the system during floods. Tides, storm surges and inlet openings are also important factors changing the hydrodynamic characteristics of the system in these extreme conditions. In the dry season, the most important factors influencing the hydrodynamic characteristics of the system are tides, sea level rise and inlet openings. Tidal water level, river floods, and sediment transport are the most sensitive acting forces influencing the stability of the inlets. The stability situation of the Thuan An inlet is in a "fair to poor" situation, according to Brunn's P/Mrot criterion. The Tu Hien inlet, which is relatively independent with the openings of other inlets, is always in a "poor" stability condition. Beside of using the model for hydrodynamic simulation of the whole system, it is recommended to employ a morphologic model (preferably 2D) in the successive steps of the study for in detail simulation of the inlets and their vicinity taking into account of effects of tides, waves, river flows, flow circular by wind, density current, sediment transport. The relevant processes and related data are also recommended for future studies.