Treatability in the coagulation and sedimentation of turbidizing substances varies greatly depending on the rapid stirring strength GR. It is demonstrated that when PACl coagulant is used, the optimum treatability is achieved at GR values of 450 for turbidity and 1,000 sec−1 for STR, a newly introduced evaluation parameter. These values are much higher than the values typical of many water purification plants where the GR values are set at 100~200 sec−1. The remarkable improvement of coagulation and sedimentation treatment is found to be related to the formation of denser, smaller-diameter microfloc with increasing GR, and an increase in the incorporation of coagulant-derived aluminium in the microfloc and zeta-potential of the microfloc, among other flock properties.