Purpose: Trunk muscle weakness in the children with cerebral palsy can lead to postural and alignment problems, breathing difficulties, and so on. Therefore, children with cerebral palsy can benefit from exercises that strengthen the muscles in their trunks. The purpose of this study was to investigate the effects of trunk muscle strengthening exercise on functional gross movement and balance ability in children with spastic diplegic cerebral palsy. Methods: We used single group repeated measure design in 8 children(four males, four females; aged 6~12 years; mean 8.3 years) with diplegia. The functional gross motor outcome measured by using the GMFM and balance ability of all children was measured by pediatric balance scale. All participants were alternately received trunk strengthening exercise and neurodevelopment treatment for 40 minutes twice per week during 8 weeks. Results: Significant and clinical meaningful improvement in functional gross motor and balance ability were shown. Conclusion: The results indicate that trunk strengthening exercise has a positive effect on both functional gross motor and balance ability in children with spastic diaplegic cerebral palsy.