This study evaluated the effect of material type, thickness and composite-composite interfacial adhesion on the load-bearing capacity of the bilayered composite structures. Bilayered cylindrical specimens (diameter=7 mm, height=5 mm) were prepared having three surface-layer thicknesses (1, 1.5, 2 mm) of Gænial Posterior to overlay base-composites [everX Posterior or Smart Dentin Replacement (SDR)]. Adhesion between surface and base composite-layers was based on an O₂-inhibited layer for optimal-adhesion (OA) or polished surface for deteriorated-adhesion (DA). Specimens (n=10) were light-cured before static-loading with a steel-ball until fracture. Stress distribution in the same designs of mechanical test was calculated by finite element analysis (FEA). Static-loading demonstrated that in both base materials, thickness and adhesion had an effect on load-bearing capacity (p<0.05). FEA showed less strain distribution for everX groups than SDR.With OA, everX Posterior demonstrated higher load-bearing capacity even with thin surface-composite layer. DA made the load-bearing capacity more sensitive to surface-layer thickness.