A combination of ultrasonication and silica gel seed was studied as a silica scale prevention method in geothermal water. The effects of ultrasonic frequency, initial monosilicic acid concentration, pH, concentration, particle diameter and pore size of the silica gel on the removal ratio of silicic acid were examined. For comparison, stirring without and with silica gel, and ultrasonication with the addition of hydrogen peroxide or t-butanol were performed. Ultrasonication enhances the removal of silicic acid by silica gel seed. Ultrasonication at 500 kHz was more effective for the removal of silicic acid than ultrasonication at 28 kHz and stirring. The removal ratio of silicic acid increased with an increase in silica gel concentration. The small seed particle diameter of 60 mu m and large seed pore size of 7 nm performed well in the removal of silicic acid. When the initial concentration of monosilicic acid was 1.1 g/L and the silica gel concentration was 1.5 g/L, the removal ratio of silicic acid was highest at pH 6 because the reaction between polymer and polymer is dominant. However, in the case of an initial monosilicic acid concentration of 0.5 g/L, the removal ratio was highest at pH 9 because reaction between polymer and monomer is dominant. The polymerization rate of monosilicic acid was increased by the addition of hydrogen peroxide due to a higher formation rate of hydroxide radicals. In contrast, the addition of t-butanol lowered the polymerization rate of monosilicic acid because the hydroxide radicals were scavenged. The frequency switching from 500 kHz to 28 kHz at 90 min for each ultrasonication enhanced the removal ratio of silicic acid.