PURPOSE: The purpose of this study was to examine the effect of using a suspension device for arm reaching activity on trunk stability and gross motor function of children with spastic diplegia cerebral palsy. METHODS: The subject in this study consisted of 11, GMFCS(Gross Motor Function Classification System) III~IV children with spastic diplegia cerebral palsy, all of whom agreed to participate in the study. All subjects were divided into two groups: the experimental group using a suspension device, and the control group using no suspension device. For each group, a thirty-minute intervention was done twice per week during 8 weeks. Before and after intervention, each test was measured using TIS(Trunk Impairment Scale), GMFM (Gross Motor Function Measure) and PRT(Pediatric Reaching Test) to change trunk stability, gross motor function and arm reaching activity. The data were analyzed with the Wilcoxon signed rank test. RESULT: All two groups had a meaningful increase in GMFM-Sit data measured before and after intervention. The experimental group had a significant increase from an average of 78.83 to an average of 84.83 in GMFM-Crawling. For both groups, there was a substantial increase in the change in sitting position and arm reaching. CONCLUSION: According to the results of this study, the arm reaching activity using suspension device had an effect on trunk stability and gross motor function and it changed arm reaching activity.