Abstract: An improved zigzag method is proposed for an optimum sizing design of trusses subjected to stress and displacement constraints. The member sectional area required for stress constraints are determined by the maximum internal force of members. The member sectional area required for displacement constraints is determined by fully utilized design criteria, the member sectional area ratio for displacement and stress constraints is calculated respectively, and the greater one of them for which loosing factor is crowned, is used to upgrade a design solution. The optimizing for a 3-bar’s and a 10-bar’s plane truss by the proposed method shows that the proposed method have properties of good stability, quicker convergence, better optimization solution and easy use.