Abstract: The fictitious gas design method for supercritical wings is used with viscous-inviscid interaction. Discussion of the effect of fictitious gas parameter and performance at off-design condition have been made for a selected baseline wing. Results show that effect of fictitious gas parameter is similar to that in 2-D flow and better performance is obtained for certain values of the parameter.Good performance at off-design condition can be obtained if Ma is not far from MaDES.