Graphene, flat carbon nanosheets, has generated huge activity in many areas of science and engineering due to its unprecedented physical and chemical properties. With the development of wide-scale applicability including facile synthesis and high yield, this exciting material is ready for its practical application in the preparation of polymer nanocomposites. Here we report that nanocomposites based on fully exfoliated graphene nanosheets and poly(vinyl alcohol) (PVA) are prepared via a facial aqueous solution. A significant enhancement of mechanical properties of the graphene/PVA composites is obtained at low graphene loading; that is, a 150% improvement of tensile strength and a nearly 10 times increase of Young's modulus are achieved at it graphene loading of 1.8 vol %. The comparison between the experimental results and theoretical simulation for Young's modulus indicates that the graphene nanosheets in polymer matrix are mostly dispersed randomly in the nanocomposite films.