Numerical investigations for a 3-dimensional WIG effect vehicle with DUP (direct underside pressurization) have been performed. The purpose of this study is to analyze the aerodynamic characteristics and static height stability of DUP using computational fluid dynamics (CFD). When a WIG effect vehicle accelerates to take off on water, increased pressure under the fuselage by DUP (propeller and air chamber) can considerably reduce the take-off speed and thus minimize the effect of the hump drag which is one of the technical difficulties of a WIG effect vehicle. The accelerated air by the propeller enters the air chamber through a channel in the middle of the fuselage resulting in an augmentation of the lift by changing the air pressure from dynamic to static. However, the DUP is not favorable for both stability and aerodynamic performance of the WIG effect vehicle because the accelerated air produces an excessive drag, negative pitching moment and 3-dimensional effects (that is, yawing and rolling moments). The result shows that the effect of yawing and rolling moments is not serious.