To evaluate the water quality characteristic after construction of Keum river estuary barrage, water quality analysis were conducted on August October in 1995 and January, May in 1996 respectively. The results were summarized as follows. The concentrations of COD were in the range of 1.01~5.10 (mean 2.50)mg/$ell$ for surface water and 0.51$~$6.68 (1.88)mg/$ell$ for bottom water. The concentrations of dissolved inorganic nitrogen (DIN) were in the range of 1.26$~$105.91 (29.66)$mu$g-at/$ell$ for surface water and 1.42$~$68.38 (19.12)$mu$g-at/$ell$ for bottom water. The concentrations of phosphate phosphorus were in the range of ND$~$0.99 (0.34)$mu$g-at/$ell$ for surface water and 0.17$~$1.04 (0.49)$mu$g-at/$ell$ for bottom water. The nitrogen ratio to the phosphorus were as high as 3.5$~$849.5 (146.5). Therefore, Phosphate phosphorus was playing an important role in phytoplankton growth as limiting factor in Keum river estuary. The correlation coefficient of salinity and DIN according to COD was shown to -0.757 and -0.874 respectivity. Mean values of eutrophicaton indies were calculated to 9.7, 7.2 for surface and bottom water, these values were exceeding 1, the value of eutrophication criteria. Especially station 1$~$3 were shown over 10 as eutrophication indices. Therefore, Keum river estuary could be evaluate to possibility area for breakout of red tides.