The purpose of this study was to investigate the activity of the transverse abdominal muscle resulting from changed posture by measuring the thickness of the transverse abdominal muscle in a supine posture, a slouched sitting posture, and an erect sitting posture. The subjects of the study were 28 patients with cerebral palsy. All their transverse abdominal muscles at the end of inhalation were measured at supine, slouched sitting (S sitting) and erect sitting (E sitting) postures by using ultrasonography, and then their dynamic sitting balance was measured at S sitting and E sitting postures by using BioRescue. For the statistical analysis, the Kruskal-Wallis test and the Wilcoxon signed-rank test were used to compare the differences among each the postures. The results were as follows. The thickness of the transverse abdominal muscle when comparing the supine posture and the S sitting posture showed no statistically significant difference. But the E sitting posture showed a statistically significant difference as compared with the others. In addition, the dynamic sitting balance in comparing the S sitting and E sitting postures showed a significant difference. In conclusion, the E sitting posture has a more positive effect on postural control and balance than generally taking the S sitting posture, for the sitting posture of a patient with cerebral palsy. It is suggested that patients with cerebral palsy mainly experiencing a sedentary life or being in a wheelchair should be seated in the E sitting posture during their daily life, and it may be necessary to continue to monitor and manage the proper E sitting posture.