BACKGROUND: IASTM is done with ergonomically planned instruments that detect and treat fascial limitations, reassure rapid localization and effectively treat parts exhibiting soft tissue fibrosis, chronic inflammation, or degeneration. Flat foot is a compound pathology defined as collapse of medial longitudinal arch of foot. It is an advanced deformity leading to various deformities of ankle and foot. AIM: the present study was taken for comparison between the effect of instrument assisted soft tissue mobilization with conventional physiotherapy treatment vs. conventional physiotherapy treatment in patients with flexible flat foot on ankle range of motion and foot posture index. METHOD: The study was a comparative study where effect of instrument assisted soft tissue mobilization was given along with conventional physical therapy in randomly classified into two groups, IASTM along with conventional physiotherapy (group A) and conventional physiotherapy (group B). Total 32 samples were given treatment for 4 weeks and the data was analysed based on foot posture index and ankle range of motion and the results were analysed using paired and unpaired t test. RESULT: the comparison of post intervention score of group A and group B was extremely significant (p = 0.0001). also, the comparison for ankle dorsiflexion, eversion and inversion of group A and group B was extremely significant. The comparison of post intervention of group A and group B for plantarflexion was however not significant. (p = 0.124) CONCLUSION: four weeks of IASTM technique has shown to improve foot posture index (FPI), dorsiflexion, inversion and eversion in flexible flatfoot in young individuals. Thus, 4 weeks of IASTM technique can be used for treatment of flexible flatfoot patients.