Introduction: Obesity is one of the major health problems in children due to changes in lifestyle and diet. Obesity in children and adolescents is associated with stature growth, and bone age. In addition, children who are under- or overweigh may be related to changes in tooth eruption. This study aimed to determine the relationship between body mass index (BMI) and number of permanent teeth in children. Methods: This descriptive cross-sectional study was performed preschool to investigate the 5 to 6 years old children in Sari city from May 2019 to June 2019. The date of birth of children was obtained from the school records on the examination day. The necessary explanations about the study are given to participants and after their consent, the examinations were carried out. SPSS software version 24 was used for statistical analysis and the significance level was considered p<0.05. Findings: A survey of 483 children in preschool centers in Sari showed that BMI had a range of 11.1- 27.6 with a median of 15.4 (14.4-16.9). The age, height, weight and BMI were similar between the two sexes (p> 0.05). According to Phakala criterion, there was no significant difference in the number ofpermanent teeth and first molars between girls and boys (p<0.05). Also, the value of dmft was not significantly different between boys and girls (p <0.05). There were statistically significant differences in count of permanent teeth according to the main criterion, dt and mt between the different weight groups (p=0.047). In addition, there was a significant correlation between BMI and number of permanent teeth, and dmft value (p<0.05). Conclusion: The results of this study showed that permanent teeth may erupt earlier in obese children, which clarifies the need for periodic dental examinations in this group of children. Also, BMI correlates with permanent teeth eruption and dmft value, so that the more weight gain may be results in the more eruption of permanent teeth and lower dmft values.