Purpose <p>This computational framework provides a predictive design tool that directly links nano- and microscale reinforcement parameters to the tunable macroscale performance of hybrid composites for advanced applications.</p> Methods <p>A multiscale modeling framework is presented that sequentially bridges the nanoscale, microscale, and macroscale to evaluate the static and vibrational responses of Al₂O₃ nanoparticles (NPs)/SiO₂ microparticles (MPs)-reinforced polyimide composite beams. At the nanoscale, a finite element micromechanics-based homogenization approach is employed to predict mechanical properties of the representative volume element (RVE), which consists of nano-Al₂O₃, polyimide matrix, and the interphase region. These nanoscale-derived properties are propagated to the microscale RVE, where SiO₂ MPs act as reinforcement within the nano-Al₂O₃/polyimide matrix. After obtaining effective microscale properties, macroscale finite element beam simulations are performed under clamped-free and clamped-clamped boundary conditions.</p> Results and Conclusion <p>Results demonstrate a synergistic effect of multiphase reinforcements, leading to increased natural frequencies and reduced deflections. Smaller NPs and a thicker, stiffer interphase are shown to enhance natural frequencies and suppress deflections of the nano-Al₂O₃/micro-SiO₂/polyimide beams. The study investigates the influence of interphase characteristics, identifying threshold ratios beyond which additional changes have negligible effects. This work offers new insights into the coupling between nanoscale reinforcement characteristics and macroscale structural performance.</p>