Background and Purpose: The purpose of this study was to find the effects of upright sitting posture on phonation and postural muscle activity in children with spastic diplegia. Subject and Methods: Nine ambulant children with spastic diplegia were chosen. The method of this study was that the children sustained vowel phonation and the vowel qualities such as FO, ]itt, Shim and NHR were analyzed by MDVP. As well, the surface EMG was used to get the muscle activity data from the pectoralis major, stemocleidomastoid, upper trapezius and rectus abdorminalis, which showed the effect of upright sitting posture. The collected data were analyzed by Wiicoxon signed rank test between pre-test and post-test and by linear regression between the vowel qualities (FO, ]itt, shim and NHR) and RMS (root mean square) of each muscle from SPSS!PC Results: The findings of this study were as follows; First, there were no significant differences of FO, ]itt and Shim between pre-post test, but there was significant difference of NHR (p<O.OS). Second, RMS of stemocleidomastoid, upper trapezius and rectus abdorminis were not significantly different between pre-post test. However, RMS of pectoralis major showed statistically difference (p<0.05). Third, we did not find the linear relationship between the vowel qualities (FO, ]itt, shim and NHR) and RMS of each muscle in pre-test (r2=0.86, 0.18, 0.37, 0.50, p>0.05) and post-test (r2 0.33 ， 0.64, 0.82, 0.49, p>0.05). Conclusions: The results suggest that upright sitting posture decrease compensatory pattems which are hyperactivity of muscles in the upper extremity, especially pectoralis major; therefore, it is an effective solution of the problem for the phonation in children with spastic diplegia.