The aim of this study is to clarify the effect of relative interfacial affinity on the formation of glass fiber-polyamide 6 (GF-PA6) networks in different polymer matrices. The fiber-polymer interaction was estimated by the contact angle of the in situ formed micro-drops on the GF surface and by the interfacial shear strength (IFSS) between the glass fiber and the polymer. The self-welded GF-PA6 network in the ternary composites was examined via the scanning electron microscopy (SEM) and dynamic mechanical analysis. It was found that a relatively high interfacial affinity between GF and PA6 (a positive Delta W*(A)) is essential for the preferential adsorption of PA6 on the GF surface. However, the encapsulation ratio, N-PA6, a parameter for the percentage of the PA6 phase that encapsulates the fiber, depends on the interfacial tension between PA6 and the polymer matrix. A simplified theoretical analysis elucidated that a lower interfacial tension between PA6 and the polymer matrix is effective to minimize the total free energy in the ternary system by increasing N-PA6 for the construction of a stronger GF-PA6 network throughout the polymer. Polymer viscosity affects the kinetic process of the preferential segregation and the PA6 domain size, which is another factor that causes the variation in N-PA6. (C) 2013 Elsevier Ltd. All rights reserved.