Graphene, a single layer of graphite, has recently attracted a large amount of attention because of its extremely high electronic and thermal properties, as many nanoscale materials are based on individual graphene. Graphene oxide (GO), which is the intermediate during the chemical processing of graphene, consists of graphene functionalized with oxygen-containing functional groups that imparts the desirable solution-processability to the neat graphene. Herein, poly(vinyl alcohol) (PVA), a hydrophilic polymer, was selected as the matrix, and PVA/GO nanocomposites were prepared by a simple and environment friendly process using water as the proceeding medium. In the PVA matrix, GO was exfoliated and nanodispersed. We found that the nanocomposites constructed by the incorporation of GO up to 1% by weight possess remarkable properties, such as significantly high mechanical and thermal properties. These excellent reinforcement effects were achieved not only by the rigid structure and high aspect ratio of the exfoliated GO but also by the strong interaction between PVA and GO. Furthermore, owing to the sheet-like structure of GO, the barrier properties of the nanocomposites were found to be dramatically increased. Polymer Journal (2012) 44, 1056-1063; doi: 10.1038/pj.2012.58; published online 18 April 2012