Abstract: Flat foot is the most common which has been found in children and adolescents. Pediatric flatfoot is categorized into flexible flatfoot and rigid flatfoot. The purpose of this study was to compare Arch Height Index (AHI) between sides and gender and to classify the Arch Height Flexibility (AHF) of medial longitudinal arch of foot based on a large foot structure data set from the 6-15 year old children and adolescents in the seven eastern provinces of Thailand. The participants were 2,909 children and adolescentsin the eastern of Thailand. Two thousand five hundred and thirty five individuals were included in the study after erroneous data were eliminated from the data set before analysis. AHI, defined by dorsum height divided by truncated foot length, was measured in three conditions: 10% of weight bearing in sitting, 50 and 90% of weight bearing in standing. AHF was calculated and classified into five categories; very stiff, stiff, neutral, flexible and very flexible. Independent t-test was used to determine the differences between gender and Paired t-test was used to determine differences between sides. The results revealed significant difference of all AHI values between boys and girls in both feet and Lt.foot was significantly lower than that of Rt. foot. For AHF, the medial arch flexibility in children was flexible to very flexible in the high percentage. This would provide a cross-sectional data which need to be considered the prevention of injuries or any consequences.