Abstract: An unsteady aerodynamic environment and highly flexible rotating blades lead to harmonic loadings on the rotor so as to cause high vibration in the helicopter.An optimization is carried out for a four-bladed,soft in-plane hingeless rotor consisting of a single-cell composite box-beam spar for helicopter vibration reduction.The design variables are the ply angles of the box-beam walls;the objective function is of the vibration hub loads.Constraints are imposed on blade rotating frequencies and aeroelastic stability.As compared to the initial design,the optimum solution results in a 20-70% reduction of the 4/rev hub loads.