This paper presents a strategy using a high fidelity delayed-detached-eddy simulation (DDES) to simulate 3-D fluid-structural interaction for predicting AGARD Wing 445.6 flutter boundary in a transonic flow. A dual-time step implicit unfactored Gauss-Seidel iteration with Roe scheme are employed for the flow solver. The special numerical techniques used herein include: a 5th order WENO scheme for the inviscid fluxes, and a fully conservative 4th order central differencing scheme for the viscous terms. A modal approach is used for the structural response. The flow and structural solvers are fully coupled vis successive iterations within each physical time step. The method is implemented as a massive parallel solver in a MPI environment. The computed flutter boundary of AGARD Wing 445.6 in a transonic flow agrees well with the experiment and the prediction made by other method.