Flat foot is the most common foot disorder that influences the alignment of the lower limb structure. It is controversial whether the use of foot insole influences kinetic and kinematic of the leg or not. Therefore, this study investigated the influence of foot insole on the gait performance in subjects with flat foot disorder. A group of flat foot subject was recruited into this study (the number of subjects was 15). The motion of the leg joints was determined using the Qualysis motion analysis system. Moreover, the force applied on the lower limb was recorded by a Kistler force plate. The range of motion of the lower limb joints, the moments applied on the lower limb joints and force transmitted through the leg were the parameters used in this study. The difference between these parameters during walking with and without insole was evaluated using the paired [Formula: see text]-test. Significant value was set at [Formula: see text]. There was no significant difference between the range of motion of ankle joint while walking with and without insole. However, the medial directed force applied on the leg decreased significantly [Formula: see text]. The use of foot insole did not influence the moments transmitted through the hip and knee joints. The walking speed of the subjects improved while walking with foot insole. Use of foot insole influenced the magnitude of the force applied on the leg and the adductor moment of ankle joint due to its influence on foot alignment. As the walking speed of the improved subjects follows the use of insole, it can be concluded that it may have a positive effects on the performance of flat foot subjects.