Aims: The aim of this study was to determine fluoride levels in drinking water of Mianeh city and to evaluate decayed, missing, and filled teeth (DMFT) index in children between 6 and 9 years old. Materials and Methods: This cross-sectional research was carried out on all sources of drinking water including 14 groundwater sources in Mianeh city. A total of 56 samples were taken from all wells during four seasons of 2009 and analyzed with colorimeter "SPADNS" method with DR 2500 spectrophotometer set17. Results: The results show that the means of source fluoride concentration in spring, summer, autumn, and winter seasons were 0.295 0.039 mg/l, 0.47 0.17 mg/l, 0.48 0.18 mg/l, and 0.4 0.06 mg/l, respectively. This means that the concentration of fluoride in all sources was below national standards, but they were in permissible range proposed by WHO. In addition, the DMFT index of the children between 6 and 9 years old in Mianeh city was higher than national mean. Conclusions: Considering the low fluoride concentration in water sources of Mianeh, it was proposed that a chemical compound such as sodium fluoride be added to provide fluoride ion. It seems that the low fluoride levels in supplied water have a significant effect on higher DMFT index in children of the subjected community.