A kind of molecular-level dispersed and highly oriented graphene monolayer nanocomposite film was successfully obtained by in situ reduction of phenyl isocyanate functionalized graphite oxide (RPIGO) in N,N-dimethylformamide in the presence of polystyrene (PS). Atomic force microscopy and transmission electron microscopy results show that the RPIGO monolayers were not only homogeneously intercalated into the PS matrix but also arranged parallel to the surface of the nanocomposite films. Because of the efficient interaction between the graphene monolayers and PS matrix, the mechanical properties of the graphene-based nanocomposite films improved significantly. Compared with the pure PS film, a 28.4% increase in the Young's modulus and a 27.8% improvement in the tensile strength of the RPIGO-PS nanocomposites films were obtained with the addition of only 0.5 wt % graphite oxide. The glass-transition temperature and onset degradation temperature of PS also increased from 96.6 and 427 degrees C to 103.2 and 439 degrees C, respectively. The improvement of the properties was mainly due to the large lateral thickness ratio and the high orientation of graphene monolayers. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 3167-3174, 2011