In this paper, characteristics of hydraulics and water quality in Kojima Bay are discussed based on field observations. The main results obtained are as follows. (1) The variation of flow rate in Kojima Bay has periods for 30 minutes to 2 hours. This characteristic is represented by a two-dimensional numerical simulation fairly well. (2) The local distributions of salinity and suspended solids show increase in the direction to the mouth of Kojima Bay. On the other hand, the distribution of COD(Mn) is nearly uniform in the whole area of the bay. Salinity appears to be influenced by river discharge sensitively but suspended solids does not. (3) The vertical distributions of salinity and suspended solids are similar. There are similar points between the cross-sectional average of water quality and surface water quality. (4) The surface water quality seems to be influenced by the variation of velocity.