Electrospun polyamide PA 66 nanofiber bundles were mechanically strengthened via compositing with polyvinyl alcohol (PVA). The highest tensile strength (126.4MPa) was achieved for the bundles immersed in the solution with 5.0wt% PVA, indicating sufficient PVA filling between nanofibers after drying the impregnated nanofiber bundles. The disappearance of voids for the bundles immersed in the PVA solution indicated a superb interface adhesion, which was responsible for the excellent mechanical properties of the composite bundles. Furthermore, the step-cycle and stress relaxation tests were carried out to shed light on the deformation evolution and the viscous stress of the bundles. The results further demonstrated the excellent mechanical properties of the composite bundles.