The effect of stream-wise fences on the twin-tail buffet responses of generic fighter aircraft is presented.  The generic fighter aircraft consists of a sharp-edged delta wing of aspect ratio one and swept-back, flexible, vertical twin tail of taper ration of 0.23.  Twin streamwise fences are located at the 30% chord-station of the delta wing.  The fences are used to alter the vortical flow and vortex breakdown location above the delta wing in order to alleviate the twin-tail buffet.  The effect of the geometrical shape of the fences and the performance of the fences over wide range of angles of attack are investigated.  This complex multi-disciplinary problem is solved using the multi-disciplinary computing environment (MDICE).  The results indicated that the fences are effective in reducing the aeroelastic loads and responses, especially at angles of attack less than 30°.  The fences of taper ration of 0.7 produced the most favorable results.