Abstract The Tanshui river is the second largest of the rivers in Taiwan, including three major tributaries, Da-han stream, Shin-dian stream and Keelung river. The large fresh water outflow induced strong stratification of the estuary and coastal region. In this region of fresh water influence (ROFI), the flow is dominated by barotrophic tide, with modification of density induced baroclinic flow. This research tries to explore the variations of vertical structure of tidal current. The causing of the changes will be discussed based on hydrodynamic parameters. Data used include a year long bmADCP records and sea level elevations. The sampling period is Septemper 24 of 2003 to July 12 of 2004. The sampling site is 4km offshore of river mouth. Additional data are winds, precipitation, solar radiation (from CWB) and river discharge (from RMO). The results of analysis show that (1) strong flow during spring may induce large tidal ellipse, while the ellipticity degenerates during neap. (2) Tidal ellipticity increases when large freshwater discharge which induced strong stratification at the surface 1~ 2 meters. (3) Wind is a minor factor of ellipticity, even during strong wind over 10 m/s. (4) Influence of solar radiation and seasonal variation of tidal ellipticity is insignificant. In summary, the analysis indicates that the vertical structure of tidal current is modified by (1) spring- neap tide (2) freshwater discharge and (3) winds. Especially, during spring tide with large river outflow, the tidal ellipticity increase significantly due to frictional effect and vertical stratification.