Flatfoot is  is a postural deformity relating to the collapse or flattening of the medial longitudinal arch. It often appears with symptoms and varying degrees of deformity and disability. Flatfoot can be influenced by two factors; internal and external factors. The purpose of this study is to estimate the correlation between body mass index (BMI) with the prevalence of flatfoot in primary school children in Laweyan. The method of this study was an analytic observational study with cross sectional approach. A sample of 285 subjects was obtained through the sample size calculation with the research formula for the proportion of one group. The subjects were 68 students of SDIT Insan Mulia and 217 students of SDIT Nur Hidayah which obtained by stratified random sampling. Inked foot print test and Chippaux Smirak Index were used to identify the flatfoot. There were 172 people (60.4%) experienced flatfoot with the highest number at flatfoot grade 1 (82%). The number of flatfoot was highest in normal BMI (58.14%) followed by obese (21.5%). There was correlation between BMI and flat foot with p<0.05. The conclusion of this study is there was correlation between BMI with flatfoot where children with higher body mass tend to have flat foot.