Background and Aims: FLATFOOT is a structural disorder in which the height of medial longitudinal arch are declined and subsequently causes biomechanical changes in FOOT. The common treatment method is the use of medial soles. This study aims to compare the effects of medial soles on Ground Reaction Forces (GRFs) during walking in male athletes with a flexible FLATFOOT. Methods: Participants were 15 male athletes with flexible FLATFOOT (Mean± SD age= 21. 33± 3. 39 years, Mean± SD height=178. 72± 5. 28 cm, and Mean± SD weight= 71. 76± 8. 04 kg). Vertical and anterior-posterior GRFs were evaluated during walking in 3 conditions (bareFOOT, shoe without sole, and shoe with sole). Data analysis was performed in SPSS software using repeated measures ANOVA with and Bonferroni post hoc test, considering the significance level at 0. 05. Results: GRF changed when using shoes with medical soles at both vertical and anterior-posterior directions compared to two other conditions, such that as the heel raised from the ground, GRF increased (P=0. 00) and the vertical GRF decreased at mid-stance phase (P=0. 02). In comparing the anterior-posterior GRF in three conditions, results showed that the posterior force was significantly higher when using shoes with medical soles compared to the bareFOOT condition (P=0. 001), while the difference was not significant for the anterior force (P=1. 16). Conclusion: By controlling the amount of pronation and eversion in the FOOT while walking using sandals with custom medical soles, the pattern of reaction force distribution can be changed at the vertical and anterior-posterior directions compared to the bareFOOT condition, indicating the effect of using a custom medical sole on the force distribution in the sole of the FOOT and subsequently on other joints. Therefore, it is recommended to study the kinetic changes of the joints following the use of medical soles.