Abstract: A theory of viscous-inviscid interaction flow for incompressible, laminar, two-dimensional case is presented in this paper. Main points of this theory are as follows. 1) the interaction flow can be divided into three layers, in the normal direction perpendicular to the main streamwise direction. The three layers are viscous layer, interaction layer and inviscid layer, respectively. A concept of interaction layer where the momentum transferring in the normal direction plays a leading role is introduced. 2) W...