In this study, the design sensitivity analysis is for the purpose of providing constraint derivative information for structural optimization under dynamic loads. Various existing formulations are reviewed, and the direct differentiation method is justified as the best one for design sensitivity analysis in structural dynamics. An alternative formulation for design sensitivity analysis with direct differentiation method is developed. The alternative formulation works efficiently with the reduced system of dynamic equations, and it eliminates the need for expensive and complicated eigenvector derivatives, which is required in the existing reduced system formulation. The relationship of the alternative formulation and the existing reduced system formulation is established originally, and it is proven analytically that the two approaches are identical, when the transformation is exact, i.e, when all the modes are included. The alternative approach is accurate, simple, and efficient. Eigenvectors are used as the base vectors in system reduction for both dynamic response analysis and the design sensitivity analysis. Lanczos algorithm is Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.