Load path analysis provides important insights into structural analysis and design. A near-optimal solution to the structural optimization problem can be obtained by load path analysis with fast convergence. Multiple load path analysis methods based on finite element analysis have been developed in the past few decades. These methods can be categorized into stress-based methods and stiffness-based methods. Stress-based methods are prone to depict the local loading conditions, which is generally straightforward and computationally efficient. By comparison, stiffness-based methods can consistently output main load paths from the macroscopic view, but greater computing power is required to solve the internal stiffness distribution. This dissertation employs load path analysis techniques to resolve the design problems of composite and lattice structures. A comparative study of different load path methods in fiber trajectory optimization of composite structures is first conducted. The orientations of the reinforced fibers are aligned with the physically determined load paths. A novel combined method is also proposed to achieve the optimal performance of the composite panels. Subsequently, two design studies of strut-based lattice structures based on load path analysis are presented. The first study leverages the characteristics of U* load paths to tailor the unit cell geometries of body-centered cubic lattice structures. Stiffness lines and potential lines derived from the U* field are used to define the unit cell edges. The second study addresses the design problems of the lattice structures composed of anisotropic unit cells. Load flows described by pointing stress vectors are used to find the optimal direction of the local reinforcement struts. Combined with topology optimization, the graded lattice structures with variable reinforcement directions can be generated. Both lattice studies are validated by finite element simulations and mechanical tests with 3D printed specimens. Given the promising enhancement in structural rigidity, load path analysis is concluded to be a valuable addition to the existing numerical approaches in structural design and optimization.