In this study, the water resistance and mechanical properties of heat-treated polyvinyl alcohol(PVA)/cellulose nanocrystal (CNC) films were investigated. PVA is the most commonly used syntheticbiodegradable polymers owing to its excellent properties. However, the water/moisture sensitivity and relativelypoor mechanical properties of PVA limits its applications. Although heat treatment is a conventionally usedmethod to improve the mechanical strength and water resistance of PVA, the effectiveness of this method isinsufficient. Therefore, CNC was used to further improve the mechanical properties and water resistance ofthe heat-treated PVA film. PVA/CNC nanocomposites containing CNC contents of 0, 1, 3, 5, and 10 wt%were fabricated using solvent casting and subsequent heat treatment. The mechanical properties and waterresistance of PVA/CNC films were significantly improved. The tensile strength and wet strength of thePVA/CNC film with a CNC content of 5 wt% (PVA/CNC 5%) were 184.5% and 136.0% higher than thoseof the untreated PVA, respectively. In addition, the water absorption and solubility of PVA/CNC 5% were56.6% and 68.2% lower than those of the untreated PVA.