The existence of interfacial instability at the interface of multilayer polymeric flows is well known. This article is designed mainly to provide guidelines for the development of experimental techniques for the improvement of two-layer polymer products in the polymer processing industry. This is done by performing a series of tensile tests on extrudate two-layer polymer melts in order to get insight into the relation between interfacial wave and mechanical properties of LLDPE/HDPE polymer system. Observed variations of the mechanical properties have been related to the conformation of the interfacial wave so that a relationship between interface morphology corresponding to extrusion instabilities and mechanical characteristics of the interfacial strength for polymer melts extrudate is established.