In this study, halloysite nanoclay was subjected to sulfuric acid treatment and used as a reinforcing agent in polyvinyl alcohol (PVA). Halloysite is one of the natural substances that is currently recognized as having a nanotubular structure resembling carbon nanotubes. Halloysite nanotubes are 50nm in diameter, 0.5-1m in length and 15nm in inner diameter. PVA, a synthetic polymer with characteristics such as biodegradability, water solubility, and transparency, was used as the matrix. The PVA/halloysite films were characterized by tensile test, scanning electron microscopy, and optical transmittance. The tensile strength of PVA/untreated-halloysite film was decreased, although Young's modulus was increased compared with those of neat PVA film. Sulfuric acid treatment was performed on halloysite to improve its interfacial adhesion with PVA. PVA/halloysite films using halloysite treated for a short time (less than 2h) had improved tensile strength, Young's modulus and optical transmittance compared with PVA/untreated-halloysite films, however composite films using halloysite treated for longer times had reduced properties.