Poly(vinyl alcohol) (PVA) has an excellent gas barrier property but its moisture barrier property is poor due to its own abundant hydroxyl groups. In this study, alkylated graphene oxide (AGO) as a hydrophobic filler and dialdehyde cellulose nanocrystal (DCNC) as a crosslinking agent, were incorporated to improve the moisture barrier property of PVA composite barrier films. DCNC was prepared through partial oxidation of cellulose nanocrystals and the presence of its dialdehyde groups was confirmed through FTIR spectroscopy and XRD analysis. The crosslinking linkage between the hydroxyl group of PVA and the aldehyde group of DCNC was confirmed through FTIR spectroscopy. The increase in thermal stability due to ether bonding was observed with TGA. The dispersion stability of the PVA/AGO/DCNC solution was also checked from the Turbiscan analysis before coating on the poly(ethylene terephthalate) substrate film. The composite barrier film decreased oxygen transmission rate by 90% and water vapor transmission rate by 15% compared to PVA barrier film.