Effects of velocity gradient (G value) and mixing time (t) on the coagulation of particles were investigated by monitoring particle counts in a rapid mixing process. Total particle counts after mixing of 30 s at G value of 200 s(-1) were similar to the summation of clay particles and alum precipitates at alum dose of 30 mg L(-1). Although small particles (microflocs less than 5 microm) were formed within the mixing time of 30 s, macroflocs larger than 8 microm did not increase significantly until the mixing time of 60 s. However, macroflocs larger than 8 microm started to increase after mixing of 75 s and they reached the maximum counts at 150 s. On the other hand, macroflocs larger than 8 microm decreased after mixing time of 180 s due in breaks of the macroflocs, which resulted in resuspension of small particles. The rapid mixing conditions for the maximum growth of macroflocs were the G value of 200 s(-1) and the mixing time of 150 s, which confirmed the best performance of turbidity removal in jar tests. Growth and break of particles and flocs in a rapid mixing step would influence significantly on the sequential flocculation and sedimentation processes for turbidity removal.