Co extrusion composites(CTC) with core shell structure were prepared by co extrusion technology with wood/high density polyethylene based(HDPE) composites(WPC) as shell layer and poplar laminated veneer lumber(LVL) as core layer. Both of the CTC and LVL were treated with boiled frozen dry environment at one cycle, and the flexural performance and failure mode of CTC and LVL were investigated by three point bending test. The results show that the bending strength, bending modulus and bending stiffness of CTC decrease by 66%, 127% and 160%, respectively, compared with LVL at room temperature. When the core is broken, the failure time increased by 441%, because the shell of WPC preventes the crack generation and development. The boiled frozen dry treated results indicate that the CTC had better environmental resistance on flexural performance，resulting from the protection of the shell layer. It can be concluded that WPC and LVL have achieved a considerable combination effect by co extrusion.