Abstract: An experimental study of the aerodynamic forces on double-deltawings with sideslips is presented,a =-3°~42 ,B=-20°~20 , Re=1.3×106°.Force, pressure, spatial flow measurements and flow visualizationshave been carried out.Results show that the sideslip makes the join ofvortices on the windward side is delayed, but the vortex will break downearlier, on the leeward side the opposite is true. The asymetric vortexbreak down has important effects on the aerodynamic forces, and makesrolling moment unstable at high angle of attach.