The Swiss ball size may affect the training intensity in spinal stabilization exercise. However, a comprehensive study regarding the effects of ball size on posture stability still lacks. This study aimed to investigate the effects of Swiss ball size on posture control of whole body by examining the center of mass (COM) and sway of center of pressure (COP) of contact point of the Swiss ball on the ground. Eighteen healthy volunteers participated in this study. The motion analysis system two force plates and a surface electromyography (EMG) system were used for data collection. The increasing ball size increased the anterior-posterior (AP) and medial-lateral (ML) sway of COM, AP and ML sway of COP and AP and ML COM-COP inclination (p<0.05). It is harder to maintain whole body balance when the lower trunk kept at a higher position. Abdominal and back muscles had stronger contractions in smaller ball size. It seems more muscle activation of trunk was required on smaller ball, despite the requirement for maintaining its balance was less. The findings may provide references for clinicians to conduct spinal stabilization exercise while considering the effectiveness and safety of this intervention.