The purpose of this study was to evaluate and compare the biceps muscles activity at the different angle and direction of the base of support on sitting position in hemiplegic patients. The biceps muscles activity was measured at the $0^{circ},;10^{circ}$ of posterior tilt, $10^{circ}$ of anterior tilt, $10^{circ}$ of affected side tilt and $10^{circ}$ of sound side tilt of the base of support by EMG biofeedback (MYOMED 432. ENLAF NONIUS CO.) In this study, 24 out-patients were evaluated who were treated at Yonsei University Medical College Rhabilitation Hospital. This study was carried out from December 5. 1993 to March 30. 1994. In order to determine the statistical significance of result, the ANOVA, and t-test were applied at the 0.05 level of significance. The results were as follows : 1. The biceps muscles activity of the sound side was no significantly difference at the different angle and direction of the base of support on sitting position(p>0.05). 2. The biceps muscles activity of the affected side was significantly increased at the 100 of sound side tilt and $10^{circ}$ of posterior tilt of the base of support on sitting position(p<0.05). 3. There was no significantly difference in the change of the biceps muscles activity of the affected side between the affected group and the intact group of propriocetive sense(p>0.05). 4. The change of the biceps muscles activity of the affected side was significantly higer in the group of G 2 spasticity compared to that of G 1, G 1+ spasticity(p<0.05). These results showed that the biceps muscles activity of affected side was significantly increased when the base of support was tilted toward the sounde side and posterior direction on sitting position. In order to prevent the increment of biceps muscle activity, the patients must avoid to sit toward sound side and posterior tilt.