Background: Generally, muscles of children with cerebral palsy (CP) lack flexibility.Muscle flexibility is a basic element for muscle action and an important milestone in the rehabilitation program for children with CP. Instrument assisted soft tissue mobilization (IASTM) is indicated for tissue extensibility dysfunction. Objective: Toinvestigate the short-term effect of IASTM on hamstring flexibility of children with diplegic CP. Patients and Methods: This study included thirty children diagnosed as diplegic CP with mild to moderate spasticity, their ages ranged from 4 to 8 years of both genders (13 girls and 17 boys). They were assigned into two equal groups; control group received conventional physical therapy program and study group received same program of controls in addition to IASTM application on hamstrings of both sides. All participated children received session two times weekly for two successive weeks. The Knee extension angle testwas used to assess hamstring flexibility before and after treatment (immediate effect after the first session and post 2 weeks). Results: There was a significant difference in hamstring flexibility after treatment sessions of both groups compared with pretreatment. The study group was significantly improved in comparison to the control group. Conclusion: Adding IASTM to the physical therapy program was found to improve hamstring flexibility in children with diplegic CP after application of four sessions.