In recent years, algal blooms have occurred worldwide, and algae-rich water often has adverse effects on water production. The technique of algae measurement is a critical issue for adjusting water treatment processes according to the numbers of algae cells. The algae particles in the water are generally 2-200 mu m in size with only a few smaller than 2 mu m. The traditional algae measuring method is by visual observation with an optical microscope. However, traditional visual observation often needs 48 h fixing time, which makes the measurement results lag behind the needs of water production. To solve the problem, this study employed on-line optical devices to improve the efficiency and accuracy of algae measurement. A photometric dispersion analyzer (PDA) and particle counting analyzer (PCA) were jointly utilized to monitor on-line the algae concentration in natural water. Algae cells can be classified by different sizes. It was found that there was good correlation between R-PDA(2) and total algae counts in water. The PCA could quantitatively characterize the algae counts and species distribution of dominant algae species in real water. PDA and PCA could be used jointly to define on-line the characteristics of real water containing mixed algae. (C) 2010 Society of Chemical Industry