The floc formation time and order were identified with the laser method while three kinds of coagulants such as polyaluminum chloride, ferric chloride, and aluminum sulfate were tested for comparison of their coagulation process. Floc formation time of three coagulants tended to decrease with the increase of coagulant dose and turbidity. When the pH was 7, floc formation time of PAC、Al2(SO4)3 were smaller than that of other pH conditions. The minimum formation time of FeCl3 occurred when the pH was 6. The minimum dose for floc formation was 0.68, 0.88, and 1.04 mg/L for PAC, FeCl3, and Al2(SO4)3, respectively. It was also found that the floc formation order of PAC was the largest among three coagulants.