The feasibility of applying an optical monitor in the automatic control of coagulation was assessed in this study. Prior to simulation testing, a series of batch tests was performed to determine the optimum state of dosing corresponding to the monitor's output. The results indicate that floc density decreased as floc size increased under the same coagulant dosage. Moreover, ratio output of the monitor could provide an index for evaluating the change trend of floc size in flocculation and for predicting the potential of floc settling. An automatic control system was designed and connected to a laboratory-scale water treatment process. A computer in the automatic control system received all the ratio values with respect to the progressive dosing. After determining the dosage, the system terminated the function of control. This control system started performing a new test for determining another optimum dosage when the fluctuation of inflow turbidity was more than a designated value. Simulation tests demonstrated that the PDA monitor with a designed control system could be successfully applied in the on-line control of coagulation dosing. The recommended control parameters for the control system was also identified.