The purpose of study was to evaluate effects of a trunk muscle strength training on sitting balance of children with spastic cerebral palsy. Five individuals with spastic cerebral palsy(three females. two males; mean aged 6.6) participated three times a week over 6 weeks. Five individuals with spastic cerebral palsy(three females. two males; mean aged 7.0) who did not receive strength training served as control. BPM(Balance Performance Moniter) was used to measured sitting balance and EMG(electromyograph, NORAXON, USA) was used to measured activities of abdominal and erector spine muscle. The obtain result are as follows. 1. The result of this study were following that maximum perturbation area and perturbation velocity were significantly reduced strength training group compared with control group(p<.05). 2. The result of this study were following that maximum perturbation area and perturbation velocity were significantly reduced after strength training compared with pre strength training in strength training group(p<.05). 3. The result of this study were following that maximum perturbation area and perturbation velocity were not significantly reduced at interval of six week in control group(p<.05). 4. The result of this study were following that abdominal muscle activity was significantly increased strength training group compared with control group(p<.05). 5. The result of this study were following that erector spine muscle activity was significantly increased strength training group compared with control group(p<.05). 6. The result of this study were following that abdominal muscle activity and erector spine muscle were significantly increased after strength training compared with pre strength training in strength training group(p<.05). 7. The result of this study were following that abdominal muscle activity and erector spine muscle were not significantly increased at interval of six week in control group(p<.05). In conclusion, the result of this study suggest that improved sitting balance and increased activities of trunk muscle by trunk strength training in spastic cerebral palsy. The result was proposed that therapeutic approach of spastic cerebral palsy with impaired sitting balance should use trunk strength training.