Effects of mixing conditions on flocculation were investigated by using a photometric dispersion analyzer (PDA) as an on-line monitoring technique in this study. Both river water and synthetic waters of humic acid (HA) and kaolin solutions were used and polyaluminum chloride (PACl) were used as a coagulant in this study. A clear relationship between F-index and residual turbidity was observed. Residual turbidity was low at high F-index. The mixing effects were also found closely related to the floc formation. When the floc formation was governed by a combination of charge neutralization and sweep floc, rapid mixing was important, but it was not important when the floc formation was governed by the sweep floc mechanism. The coagulant dosage governed the floc size and strength in the sweep floc region. The higher the coagulant dosage was, the larger but the weaker the floc was. Rapid mixing effects were different, depending on raw water characteristics. Fast and large floc formation was observed in flocculation of the kaolin solution, compared to that of the HA solution. Small HA would be mostly adsorbed onto the hydroxide precipitate after the precipitate formed. The adsorption could retard further floc growth. The resulting floc was small, and the floc formation was slow. However, kaolin helped flocculation by bridging the hydroxide precipitates, leading to fast and large floc formation. Temperature affected the flocculation kinetics as well as the floc size. A large floc formed at high temperature. The flocculation kinetics became fast with increasing temperature.