One of the key requirements for developing multiphase polymer products is the ability to predict the fracture and tensile strength of these materials when subjected to static and dynamic loading. The existence of wave instability at the interface of multilayer polymer products is well known. The experimental studies have shown that interfacial wave instability of two-layer Poiseuille flow play an important role in polymer processing operation. This article is designed mainly to provide guidelines for the development of experimental techniques for the improvement of the two-layer polymer extrudate in the polymer processing industry. This is done by performing a series of tensile tests on two-layer polymer extrudate in order to get insight into the relation between interfacial wave and mechanical properties of high density polyethylene (HDPE) and polypropylene (PP) systems. 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 this polymer system (PP/HDPE) are established.