Investigations of oxygen barrier and blending properties of the modified polyamide (MPA) and nylon-6 clay (NYC) blends (MPANYC) were systematically investigated. After blending NYC in MPA, the oxygen barrier properties of the MPANYC film specimens are significantly better than those of the MPA and NYC film specimens. At 20 wt.% optimum content of NYC, the oxygen barrier improvement of MPANYC film specimen reaches the maximum, which is in the same order of magnitude of barrier improvement as those of PVDC film specimens. A similar relation on the NYC content was found on the plots of free free-volume properties (i.e. Rf, Vf, I3 and Fv) vs. the NYC contents of MPANYC specimens. The minimum Rf, Vf, I3 and Fv values of MPANYC specimens were found as their NYC contents reach about 20 wt.%. The characteristic X-ray reflections of α form PA crystals originally associated with the MPA resin almost disappear after blending varying amounts of NYC in MPA resins. Further morphological investigations showed that demarcated structures of elongated (ca. 700 nm) and overlapped clay mineral layers were found on the microtomed surfaces of MPANYC specimens as their NYC contents reach 20.0 wt.%. However, at NYC contents lower than 20.0 wt.%, the lengths of these “elongated and overlapped” clay mineral layers reduce significantly as the NYC contents continue to reduce. These improved oxygen barrier properties of the MPANYC film specimens were explained in terms of the reduced free-volume properties and demarcated structures of the elongated and overlapped clay mineral layers caused by the presence of the nanometer clay mineral layers.