Background: Diastema is one of the many esthetic abnormalities due to various causes. One of which is abnormal frenulum. Objective: The aim of this study is to determine the prevalence of different types of labial frenum and their effect on median diastema in 3–6-year-old children in Tehran kindergartens. Materials and methods: This study was a cross-sectional one that was performed on 639 children aged 3–6 years by referring to kindergartens. Studying these children included oral examination under normal light with a tongue depressor in upright position. By lifting the patient's lip and performing a blanching test, it was determined whether or not a person has normal frenulum. Information was also provided about the presence or absence of median diastema in upper jaw and the type of frenum adhesion. Also, the central teeth in maxilla were carefully examined in terms of spacing and caries. Data were analyzed by descriptive and analytic statistical methods. In the analytic part we used the binomial logistic regression test and also in the descriptive section of the frequency, tables and graphs are presented. Conclusion: In this research, a total of 639 children were studied, 341 of which were male and 298 were female. Among these 214 people (33.5%) had abnormal frenum and 425 people (66.5%) had normal frenum. Frenum adhesion site for 52.9% of the people was observed in attached gingiva and for 19.9% of people was observed in the mucogingival junction and for 18.8% of people was observed in the interdental papilla and for 8.5% of people the site of frenulum was in depth of the palatal papilla, and there was no significant difference between two sexes in terms of prevalence of different types of maxilla labial frenum. The results showed that there is a significant relationship between high frenum and maxillary median diastema and there is no significant relationship between age and high frenum.