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 (969 boys, 1031 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 60.8%. Off all the three groups (Group I 6-8 years old = 55%; Group II 9-10 years old = 64.2% and Group III 11-12 = 61.5 years old) with the highest caries prevalence in group III and lowest in group I. The overall mean DMFT, deft and dmft indices were 1.01(SD ± 1.48), 1.35 (SD ± 1.84) and 1.45 (SD ± 2.39), respectively for all subjects. There was a statistically significantly negative correlation in overall mean DMFT scores of children at various water fluoride levels (10.73 ± 0.36 ppm) (P = 0.020). Caries experience was more among girls than boys (P = 0.021). 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. Water defluoridation on an urgent basis is a priority here than water fluoridation, because the prevalence and severity of dental fluorosis is very high. Effective oral health promotion strategies need to be implemented to further improve the dental health of school children.