The flutter boundary of the 3D AGARD 445.6 Wing is calculated by using an efficient upwind scheme, Zha CUSP2, in moving grid system. The 3D Reynolds average Navier-Stokes equations are solved. Closure of the Reynolds stresses is provided by the Baldwin-Lomax turbulence model. The modal approach solver is used to calculate the structural response under aerodynamic forces with second-order accuracy. The CFD and structural solvers are fully coupled implicitly via successive iterations within each physical time step. The numerical results show that this method is capable of capturing the aeroelastic properties of the flexible wing efficiently. The computed flutter boundary of AGARD Wing 445.6 for free stream Mach numbers ranging from 0.499 to 1.141 agrees well with the experiment.