Abstract: In this paper, an optimization method for gradient lattice structures based on MIST is proposed. The thickness of the lattice structure rods are optimized by choosing the total strain energy of the rods as the objective function and the cross-sectional area of the rods as the optimization variable. The optimization goal is to minimize the total strain energy. The length feature parameter is used to describe the density distribution of the lattice structure. The density distribution is optimized by choosing the length feature parameter as the optimization variable. Then a concurrent optimization method with two types of variables is proposed to optimize the rod thickness and the density distribution at the same time. By optimizing the uniform lattice structure, the non-uniform gradient lattice structure is obtained. Finally 2D and 3D numerical examples are presented to demonstrate the effectiveness of the proposed method.