In order to understand the relationship between processing conditions and the properties of weld-lines on a molecular level, it is necessary to evaluate the true strength of the weld-line that is not affected by the V-shape notch on the surface of the weld-line zone. In this experiment, the weld-line strength of several brittle, ductile, or phase-separated polymers was evaluated using the compact tension test by measuring the critical stress intensity factor, K-IC, or the critical J-value, J(IC), and the results were compared with those obtained by tensile testing. For brittle polymers such as poly(methyl methacrylate) (PMMA) or styrene acrylonitrile copolymer (SAN), the value of the weld-line factor, i.e., the strength ratio between the welded and the non-welded specimen, is higher than that measured by tensile testing, because of the notch sensitivity of brittle thermoplastics and the notch dependence of tensile strength. On the other hand, in the case of ductile polymers such as polycarbonate (PC), the weld-line factor is similar for both the tensile and compact tension tests. However, the dependency of the weld-line factor on melt temperature is more obvious in the compact tension test. From these results, it seems that the compact tension test is more appropriate for measuring the interfacial adhesion strength across the weld-line, which excludes the notch effect.