Background and Purpose: The purpose of this study was to find the effects of ankle training on the trunk activity in sitting position during lateral weight transfer in children with spastic diplegia. Subjects and Methods: The subjects were 12 children with spastic diplegia who had been treated at Hospital in Bundang-gu, Seongnarn-si as inpatients or outpatients. The method of this study was that the subjects were treated 10 times for 30 minutes each from september, 2007 to October, 2007. As well, the surface EMG was used to get the musc1e activity data from the rectus abdominais ， lumbar paraspinals, tibialis anterior, and gastrocnemius which showed the effects of ankle training. The collected data were analyzed by Wilcoxon signed rank test between pre-training and post-training, by Marm Whitney U test between the dominant and non-dominant side, and by Pearson correlation for relations among RMS(root mean square) of each muscle from SPSS/Pc. Results: The results of this study were as follows; First. there were no significant differences of RMS of the lumbar paraspinalis, gastrocnemius between pre-post training in the dominant side. but there were significant differences of RMS of the rectus abdorminalis. tibialis anterior(p<0.05). RMS of the lumbar paraspinals, gastrocnemius were not significantly different between pre-post training in the non-dominant side. However, RMS of the rectus abdorminalis. tibialis anterior showed statistically differences(p<0.05). Second, there were no significant differences of RMS of the rectus abdorminalis, lumbar paraspinals, gastrocnemius between the dominant and non dominant side in the pre-training, but there was significant difference of RMS of the tibialis anterior(p<0.05). We did not find significant difference of RMS of each muscle between the dominant and non-dominant side in the post-training. Third. there were related to the lumbar paraspinals of non-dominant and gastrocnemius of dominant(r=0.641). the lumbar paraspinals of non-dominant and tibialis anterior of both side(r=0.722, 0.580), the gastrocnemius of dominant and tibialis anterior of both side(r=0.779. 0.612), each other of 디bi 외is anterior in pre-training(r=0.764). Finally, we found the correlation between the rectus abdorrninalis of dominant and gastrocnemius of non-dominant(r=0.795), the lumbar paraspinals of dominant and gastrocnemius of non -dominant(r=-0.613). the lumbar paraspinalis of dominant and non-dominant in post-training (r=0.705). Conclusions and Discussion: The results suggest that ankle training increased trunk activity in sitting position during lateral weight transfer in children with spastic diplegia; therefore, it is an effective solution of the problem for trunk inactivity and ankle instability which the children with prematurity have.