Aim: To assess prevalence and severity of dental caries among 6-12 years old children in relation to fluoride concentration levels in drinking water. Design: Cross-sectional observational study, done in two cities, Libya (endemic fluoride belt). Subjects and Methods: 2000 children; aged 6-12 years (922 boys, 1078 girls) were examined from four randomly selected public schools. Dental caries was measured using the WHO diagnostic criteria (1997).Water fluoride levels were determined from samples obtained from drinking water sources. Results: Overall caries prevalence was 65.5%. The overall mean DMFT, deft and dmft indices were 0.23(SD ± 0.64), 2.60 (SD ± 2.67) and 2.41 (SD ± 2.64), respectively for all subjects. There was a statistically significantly negative correlation in overall mean DMFT scores of children at various water fluoride levels (1.44 ± 0.05 ppm) (P ≤ 0.05). There was a statistically significant positive (direct) correlation between water Fluoride level and def. Caries experience was more among girls than boys (P ≤ 0.05). Conclusion: The caries prevalence among schoolchildren was very high, and that there was a negative correlation between caries experience and fluoride concentration for the entire study population. However, in high fluoride areas, there was a positive correlation between fluoride concentration and dental caries. Effective oral health promotion strategies need to be implemented to further improve the dental health of school children.