A decomposition strategy for the structural optimization of a fiber-reinforced aircraft wing box is proposed. Theproposed method decomposes the wing-box optimization into two levels: a system-level and a subsystem-leveloptimization. The ribs are the subsystems of the problem. Each rib has a local set of design variables andconstraints. The loads on the ribs are the crushing loads caused by the bending of the wing. At the system level,the wing-box skins are optimized while accounting for the effect of the skin design on the loads applied to the ribs. Thesensitivity of the rib mass to the applied loads is evaluated using the Lagrange multipliers of the optimized rib design.To enhance the numerical efficiency of the two-level optimization, the changes of the loads on the ribs are subjected toa reduction by principal component analysis (PCA). In both the wing-level and rib-level optimization problems, thelevel-set strategyfor the optimization of compositestructures, previouslyintroduced by the authors, is employed.Thismethod permits an advantageous use of coarse and fine finite element models employing a standard commercial finiteelement code. The proposed method is applied to the design of a composite horizontal tail plane. The accuracy of andthe computational time savings by the proposed PCA-based reduction scheme are quantified.