This paper presented construction and strain distributions for light-weight wood-fiber-based structural panels with tri-grid core made from phenolic impregnated laminated paper composites under bending. A new fastening configuration of slots in the faces and tabs on the core was applied to the face/core interfaces of the sandwich panel in addition to epoxy resin. Both normal strain gages and shear strain gages were attached on these panels to analyze inside strain distributions by third point load bending test. The purpose of the bending test was to investigate the various strain distributions of panels with different face/core configurations that identified the critical failure modes for future design. In this research, four panels with different configurations were constructed to analyze the influence of strain distributions for bending behavior. Either maximum localized normal strain or shear strain were used to judge failures and associated failure modes through observation. Test results of strain distribution showed normal strain was primarily carried by both top and bottom faces. As bending load increased, compression buckling occurred on the top surface of some panels with thinner faces. Face thickness and stiffness significantly affected the strength of the panel as evident by nonlinear strain behavior. Meanwhile, the shear strain was primarily taken by the ribs in the structural core, and shear failure always occurred in the longitudinal linear ribs of core with thicker faces. The shear strain in the cross ribs was approximately half that of the longitudinal linear ribs in the same section of shear zone, which was consistent with the geometric formula. The problem of panel imperfections resulting in either face compression buckling or rib shear buckling could be overcome by further design optimization, and the analytical modeling for bending design and evaluation was presented.