Characteristics of flocs formed by charge neutralization and sweep coagulation using titanium tetrachloride (TiCl4) were investigated with humic acid-kaolin suspension by continuous optical monitoring. This paper focused on the regrowth ability of broken flocs after addition of second TiCl4 dose. Variation of floc size and the fractal dimension of flocs versus second TiCl4 dose after regrowth were investigated. Second TiCl4 dose was added during the floc breakage period, and addition time of second TiCl4 dose was also investigated. The results showed that, when coagulated by charge neutralization at pH 6, an appropriate second TiCl4 dose improved regrowth ability of broken flocs at low initial TiCl4 doses. While for high initial TiCl4 doses, second TiCl4 dose lowered floc re-growth ability. When coagulated by sweep coagulation at pH 10, second TiCl4 dose made regrown flocs larger than those without second TiCl4 dose. Floc structure analysis showed that it was determined by not only the fractal dimension of flocs, but also the chemical characteristics of floc surface. Addition time of second TiCl4 dose had a great effect on floc regrowth ability, suggesting that the broken flocs had better regrowth when second TiCl4 dose was added at the end of the breakage period. (C) 2011 Elsevier B.V. All rights reserved.