Purpose: The present study investigated the most appropriate footprint index and its related parameters for the diagnosis of flatfoot, especially in cases with mild deformity. Methods: In total, 148 footprints were recorded from 77 young women with flatfoot. Footprints were classified into three groups of mild, moderate, and sever. Footprint indexes included Sztriter-Godunov (KY), Clarke’ s angle (CA), Chippaux-Smirak Index (CSI), and Staheli (ST) indexes. The mentioned indexes and their relevant parameters were calculated on the footprints in the aforementioned groups. Exploratory factor analysis was used to identify proper components for the diagnosis of flatfoot. By multivariate analysis of variance, between-group differences were assessed. A Chi-squared test was used to identify the most accurate index for the diagnosis of mild flatfoot. Results: Exploratory factor analysis based on the parameters of footprint indexes identified three essential components. Two identified components were based on the 6 parameters of CSI, KY, and ST indexes in moderate and sever footprints. The third component was related to footprints with mild grading and was composed of three extracted parameters from midfoot region. Moreover, the linear combination of the three last parameters suggested a significant difference between three groups (P<0. 01). Eventually, KY index could identify the highest number of flatfoots in the mild group (P=0. 000). Conclusion: Study findings revealed that indexes in which parameters were extracted from midfoot region could be suitable for the diagnosis of mild flatfoot. KY index’ s parameters were extracted from midfoot. Considering that KY could identify several remarkable mild flatfoots in comparison with other indexes, we can introduce it as a proper index.