Coagulation and flocculation are widely applied in water and wastewater treatment. Removal of suspended particles is essential in water treatment and greatly depends upon the performance of the coagulant and the production of flocs with suitable properties. For monitoring floc size, formation, and size transformation, no particle size method now can be considered ideal. In this work an on-line laser particle counter was used to follow coagulation with aluminum sulfate. The experiments showed that the floc formation and floc size distribution could be well monitored. The results showed that it is feasible to use particle counting for dosage control and for monitoring changes in floc formation, transformation and size distribution.