Aims: to determine the prevalence of caries of the first permanent molars (FPMs) and to compare the caries incidence between maxillary and mandibular arch, and to determine the most affect segment by the decay in the dental arch.

Material and Methods: This cross-sectional survey was conducted of 375 children aged 6 to 12 years in Benghazi city of Libya. The clinical examination for dental caries was based on the World Health Organization (WHO) criteria. It has been calculated the frequency of caries free (CF) and the DMFT index in terms of decayed teeth (DT), filled teeth (FT) and missing teeth (MT). Data were analyses using SPSS version 16, Chi-square test and the Kruskal-Wallis tests were used.

Results: A significant difference was found between sound and carious FPMs in all dental arch segments (P =0.001). The DMFT index of the sample was (1.80±.081) whereas; the highest rate was in left side of the mandible (0.51±.026). The DT component was the highest rate (92.6%) of the DMFT index.

Conclusion: The DMFT in the mandible was higher than the maxilla. The caries prevalence of FPMs was similar in both sides of the dental arches, thus the intensive preventive and health service activities should be promote in Libya to decrease caries risk in school children.