The radius of gyration (Rg) of polymer chain is related to the degree of chain entanglement in glassy polymers. In this study, we investigate the effect of the Rg of polymer chain and the intrinsic interfacial strength between materials in mode I fracture of polyethylene (PE)-graphene & PE-transition metal bilayer structures each of which are weakly and strongly bonded at the materials’ interface respectively. From a quasi-static decohesion test of the bilayers using the statistical ensemble simulations, the separation distance to resultant traction relationship (Cohesive law) is derived according to the Rg of PE chains. As a result, a clear Rg dependent cohesive strength and fracture toughness at the interface is observed. Moreover, the cohesive law and the crack initiation point is found to be affected by the intrinsic strength between the tested materials.