Objective A flat foot is a complex postural deformity in which the medial longitudinal arch is decreased. It is associated with diverse symptoms and varying degrees of deformity and disability. This disorder can sometimes contribute to problems in the ankles and knee as it can alter the alignment. Conservative treatments are used to treat patients with a flat foot. One common treatment is the use of insoles with medial heel skive technique. The purpose of this study was to evaluate the effect of custom-molded insoles with new technique on pain and function in females with a flexible flat foot. Materials & Methods This research is a quasi-experimental study of before-and-after type that included 12 subjects with a flexible flat foot from the Social Welfare and Rehabilitation Sciences University of Tehran. To provide a negative cast, plaster cast impressions were taken for each participant's foot. According to the method of modification (medial heel skive) that was introduced by Kirby, the positive cast of the foot was corrected. Each person received an insole with medial heel skive technique. The pain and function of each foot were evaluated with a visual analogue scale and Foot and Ankle Outcome Score before applying the treatment and at six weeks follow-up. Paired t-test and non-parametric tests were used for statistical analysis. Results It was found that the pain and function significantly improved after 6 weeks of using insoles with medial heel skive technique in subjects with flat foot (P<0.001). Conclusion According to our findings, there was a significant difference in pain and function in patients with a flat foot through medial heel skive technique. Therefore, it seems that the insoles can be an effective prescription for subjects with a flat foot.