Bounding motion in a seated position on a G-ball^a) for children was observed with the aim of obtaining a basic knowledge of the effect of the G-ball motion on the children's sitting-posture. Dynamical and mental reflective analyses about the change in the posture before, after and during the motion were attempted. The results are as follows. a) The name G-ball comes from the first letters of Giant, Gymnastics and Gravity. 1. The average tempo of the bounding motion for all the subjects was 109±6.2 bounds/minutes. Separately, it was 108±6.4 and 111±6.3 bounds/minutes for males and females, respectively. There was no significant difference. 2. The average value of BED (Back-Ear-Distance) in the sitting posture before and after the bounding motion was, 18.3±3.71 cm and 17.1±4.04 cm, respectively. It significantly decreased (P<0.01). Separately, BED significantly decreased both for males and females, but the decrease was more apparent for males (P<0.01) than for females (P<0.05). The average values of BED before and after the bounding motion for hump-back group were 21.8±2.53 cm and 20.3±3.84 cm, respectively. Those for the middle group were 17.3±0.96 cm and 16.3±1.57 cm. Both groups showed a significant decrease after the bounding motion (both groups P<0.01). 3. Both the values of BED at the highest and the lowest positions, averaged for every 15 bounds, showed a significant decrease as the number of bounds increased (P<0.05). 4. Mental reflection investigation executed after the exercise showed that 94.1% of the children gave a favorable answer.