This study addressed the size and fractal dimensions of floes coagulated from highly turbid raw water using polyaluminum chloride (PACl) as the coagulant. Large flocs with loose interior structures arc preferred for removing turbidity and humic acid from water. The response surface method, with the Box-Behnken design of experiments, was used to elucidate the effects of pH, turbidity, alkalinity of suspension, doses of PACl, and humic acid on the characteristics of the floes. Nondimensional correlations between the floc size and the fractal dimension of the coagulated floes were derived by regression analysis. Graphical presentation facilitates the interpretation of the data obtained from the designed experiments. The variable that most affects the floc characteristics is the PACl dose. However, the "optimal" conditions that lead to large flocs are different from those that lead to loose flocs. A compromise must be made to generate satisfactory flocs from highly turbid raw water. The conclusion drawn from bench tests is applicable to extremely highly turbid stormwater obtained during storm Nari on September 16-19, 2001.