A nonlinear reduced order modeling (ROM) approach for efficient aeroelastic design/analysis using high-level CFD with a tightly-coupled structural FEM interfacing is proposed. The ROM is constructed with a proper orthogonal decomposition (POD) scheme producing low-order but highly accurate solutions over a wide range of frequencies. A novel harmonic balance technique is introduced to handle the flow nonlinearities such as limit cycle oscillations (LCO). The proof-of-concept examples indicate that the proposed frequency-domain method is at least two-orders of magnitude fast than conventional time-marching CFD methods. Next, the ROM/POD methodology is extended to include multiple degree-of-freedom in generalized structural coordinates with CFD/structural FEM grid transferal via ZONA’s boundary element method (BEM) solver. Two wings, the 445.6 wing and the PAPA wing, are selected for the validation of the 3D Euler ROM/POD method proposed in phase I. Meanwhile, a 2D nonlinear model will be dedicated to the study of transonic LCO. Finally, demonstration and merit assessment of the proposed method will be conducted with regard to its efficiency, accuracy, modularity and the understanding of key physics towards the understanding of LCO. If proven successful, the proposed method is likely to become a new-generation, efficient CFD method for transonic flutter/LCO, aeroservoelasticity and MDO applications in industry.