Objectives : The purpose of this study was to understand and compare the knowledge of the adolescents and adults among the residents of Seoul metropolitan city about water fluoridation program. Methods : A total of 613 adolescents and adults between the age of 15 and 31-49 was surveyed by the questionnaire. Among them, the source of drinking water, the source of cooking water, awareness of caries prevention effect of water fluoridation program, awareness of cost benefit of health insurance of water fluoridation program, approval of water fluoridation program, awareness of health safety of water fluoridation program were surveyed. Results : 85.5% of adolescents and 88.9% of adults were surveyed to use public water supply for drinking water, and 95.3% of adolescents and 96.8% of adults were surveyed to use public water supply for cooking water. But only 14.0% of adolescents, comparing to the 42.1% of adults, were surveyed to be aware of the caries prevention effect of water fluoridation, and only 6.0% of adolescents, comparing to the 24.6% of adults, were surveyed to be aware of the caries prevention effect of water fluoridation to reduce to health insurance expenditure. About 82.6% of adolescents, comparing to the 92.1% of adults, were surveyed to vote for the fluoridation program, and only 6.8% of adolescents, comparing to the 28.6% of adults, were surveyed to be aware of health safety of water fluoridation program. Conclusions : Extensive oral health education program to the adolescents are necessary about using water fluoridation to prevent the dental caries.