PURPOSE: The purpose of this study was to investigate the effect of trunk muscle strengthening exercises on balance performance of sitting posture and upper extremity function, targeting the children with spastic diplegic cerebral palsy. METHODS: 20 children with spastic diplegic cerebral palsy were sampled at random and the tests were conducted for 6 weeks, 3 times per week. For experimental groups, basic physical therapy and trunk muscle strengthening exercises were conducted and for control groups, only basic physical therapy was conducted. BPM(Balance Performance Monitor) was used to measure balance performance and QUEST(quality of upper extremity skills test) was used to measure the upper extremity function. RESULTS: The comparison of changes in sitting balance performance in between experimental groups and control groups show significant difference (p<.05), the changes of the upper extremity function in experimental groups and control groups show significant difference (p<.05). CONCLUSION: Trunk muscle strengthening exercises are effective in improving balance performance and the upper extremity function for the children with spastic diplegic cerebral palsy.