In this study, we optimized the curved fiber trajectories to realize variable fiber volume fraction and stiffness composites (VVfSC) using a continuous fiber composite 3D printer. During optimization, the fiber orientation was maintained along the principal stress direction based on preliminary stress field calculations, and the fiber trajectories were subsequently obtained. The fiber volume fraction was calculated from the obtained fiber trajectories; then, stress field calculations and redetermination of the fiber trajectories were performed. Optimization was achieved by repeating this sequence until convergence was obtained. Based on the optimization result, the specimens were molded using a continuous carbon fiber 3D printer and evaluated with bolt joint tensile tests. It was demonstrated that the stiffness and strength per unit weight of the optimized VVfSC were 9.4 and 1.6 times greater than those of conventional linear laminates, respectively.