The in situ intercalated polymerization of vinyl acetate and montmorillonite clays (MMT) was initiated via γ−ray irradiation. Results of X−ray diffraction (XRD) and transmission electron microscopy (TEM) indicate that polyvinyl acetate (PVAC)−montmorillonite clays (MMT) composite was a kind of nanocomposite. Scanning electron microscopy (SEM) testing result shows that the nano−particles of MMT were packaged by PVAC and the capsule particles was formed, so the MMT particles could be better dispersed in the matrix of HDPE when the polyvinyl acetate (PVAC)−montmorillonite (MMT) nanocomposite was melt−blended with HDPE. For the good compatibility of PVAC and HDPE, these capsule particles are also compatible with HDPE. Thermogravimetric analysis (TGA) results show the existence of MMT nano−particles in the HDPE is helpful to increase the thermostability of it, and the mechanical properties of HDPE were improved. Moreover, the PVAC/ MMT/ HDPE nanocomposite exhibit much better water vapor permeation resistance than HDPE.