When very soft soils are dredged and reclaimed by pump dredger, coagulants are often added to dredged material slurries in order to prevent water pollution by effluent. The settling properties of dredged material slurries which added coagulant differ from those added no coagulant. Authors carried out settling tests using actual clay and polymeric coagulant in order to clear up the effects of coagulant to the settling properties of dredged material slurries. The following main conclusions were obtained through this study. 1) In settling stage, the interface's settling velocity of the slurry is increased by adding coagulant. The coagulant adding ratio which gives aximum settling velocity is larger than that gives minimum residual turbidity. 2) The height of the deposit at the end of settling stage is decreased by adding coagulant. 3) The final height of the deposit is increased by adding coagulant.