Studies on the flocculation of algae using various metal ions were carried out by measurements of optical density(OD) and zeta potential. Cyanobacteria were used as algaes. Flocculation efficiencies of cyanobacteria by an addition of metal ions were determined from OD values, and the effect of metal ions was greater in the order of Al 3+ > La 3+ > Ho 3+ > Fe 2+ > Ca 2+. Especially for trivalent metal ions, percentages of metal removed from cyanobacteria solutions on flocculation were measured, showing the same order as in flocculation efficiencies. Zeta potentials of cyanobacteria alone were measured with increasing the concentration, found to be all negative voltages, and were increased with increasing the concentration. The effect of pH on zeta potential of cyanobacteria solution was investigated. Below pH 5.5, the zeta potentials were steeply decreased with increasing pH, whereas in the range of 5.5 ≤ pH ≤ 10 they were almost constant (-46±1 mV) even with increasing pH. At a constant concentration of cyanobacteria (A730=0.25), an increase in concentration of metal ions caused an increase in zeta potential of cyanobacteria solution, showing that the effect was greater in the order of Al 3+ > Ho 3+ > La 3+ ≫ Mg 2+ ≥ Ca 2+ ≫ K +. At a constant metal concentration, zeta potentials were measured with increasing cyanobacteria concentration, showing that zeta potentials for K + , Mg 2+ and Ca 2+ ions were negligibly changed, whereas those of Ho 3+ and La 3+ ions were decreased. Moreover, the effect of Ho 3+ ion on decreasing zeta potential was smaller than that of La 3+ ion. Al 3+ ions showed quite a different behavior that with increasing cyanobacteria concentration the zeta potentials increased and decreased thereafter. Hydrolysis of Al 3+ ions caused a difficulty to investigate coagulation or flocculation of cyanobacteria by measurement of zeta potentials.