Polymer blends considerably expand the field of application of polymer materials, but the final properties of the alloy critically depend on the morphology of the immiscible and/or miscible domains that develop in blending and processing. A variety of parameters can influence the final morphology of a polymer blend, such as viscosity ratio, composition, processing method, and types of melt flow; but the most important factors are interfacial properties. Theso-called breaking thread method was adopted to measure the interfacial tension of polymer melts. Here, surface contamination and good contact between thread and applied film are two difficult to control key points that may influence the test result. With the help of core-sheath bicomponent spinning technology, a novel interfacial tension test sample preparation and measuring method were designed, which can avoid said problems. In this study, interfacial tensions of PA6/LDPE with and without compatibilizer were studied. Results showed that the interfacial tension of PA6/LDPE decreased with the addition of a compatibilizer, and that a kind of interfacial layer formed between core and sheath, which improved the mechanical properties of the PA6/LDPE bicomponent fiber.