Background: Oral hygiene is one of the most important health issues in children that affects the health of children. This study aimed to investigate dental caries prevalence, Decayed, Missing, and Filled Teeth (dmft and DMFT Indexes) and the associated factors among 9 -11 years old children.Materials and Methods: This cross-sectional study was performed on 4226 primary school students aged 9-11 years old using the census method in Iran. Data were collected by valid and reliable researcher-made checklists. Data analysis was performed using SPSS software version 24 and using Kruskal Wallis, Mann-Whitney, and Logistic Regression tests.Results: The prevalence of permanent and primary dental caries in all students was 39.73% and 67.72%. The mean (standard deviation) of DMFT in the 9, 10 and 11 age groups were 0.6 (1.11), 0.97 (1.39) and 1.23 (1.63), respectively. The mean (standard deviation) dmft in the 9, 10 and 11 age groups were 3.14 (2.56), 2.46 (2.34) and 1.37 (1.84), respectively. There was a significant difference in DMFT and dmft between age groups (P<0.001). Results of multivariate logistic regression showed that there is a significant relationship between gender, birth rate, place of residence, mother's education, father's education and use of mouthwash with DMFT index (P<0.05). The results of multivariate logistic regression showed that the variables of insurance, residence and mother's job were significantly correlated with dmft index (P<0.05). Conclusion: In this study, the prevalence of dental caries was high. Variables of gender, birth rate, parental education, place of residence, use of mouthwash, having health insurance and mother's job were effective in predicting dental caries in students. it seems necessary to hold educational courses to increase awareness among students, educators, and parents about oral Health. Keywords: Elementary students, School, dmft, DMFT, Children, Oral health