cc This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.Purpose: The objective of this study was to evaluate the caries prevention effect of school-based fluoride mouth rinsing (FMR) program implemented in the Hanuul district of Mongolia, which has a very low socioeconomic status and extremely poor infrastructure for oral health. Materials and Methods:One hundred and seventy children aged from 6 to 8 years of the FMR school and 187 children aged from 6 to 8 years of the control school completed the baseline survey. Children from the FMR school rinsed with 0.05% sodium fluoride everyday under supervision, while those from the control school did not. Adjusted caries preventive fraction (CPF) for 2 years were calculated to evaluate the effect of the FMR program.Result: After 2 years, 288 schoolchildren remained in the study. Decayed, missing or filled permanent teeth (DMFT) and index of the FMR and the control schools at baseline were 0.11 and 0.12, respectively, and the average DMFT increment of the FMR and the control schools after 2 years were 0.35 and 0.65, respectively. The adjusted CPF of DMFT was 48.5%. Conclusion:These findings show that a school-based FMR is an effective caries preventive program in a socially deprived community with poor infrastructure for oral health.