The bending property and failure mechanism of sandwich beams were investigated. The multi-walled carbon nanotubes (MWNTs) were used as the reinforcement in the MWNTs/polymer nanocomposites and the epoxy and phenolic resins were used as the matrix. The MWNTs/polymer nanocomposites were used as the core materials of sandwich beams, which had faces made by graphite/epoxy laminates. In experiment, the hot press method was applied to fabricate the MWNTs/polymer nanocomposites and graphite/epoxy laminates. The weight percentage of MWNTs in core materials and the fiber orientation of face laminates were varied to assess their effects on the bending properties of the sandwich beams. The failure mechanism of sandwich beams with two kinds of polymer core materials was investigated. In analysis, the finite element method was used to obtain the bending behavior of the sandwich beams. The numerical results were in good agreement with the experimental ones.