The nanocomposites of polyurethane (PU)/oxidized grapheme (GO) were in-situ prepared by a two step process, i.e. firstly the oxidized graphene (GO) and 4, 4’-diphenylmethane diisocyanate (MDI) were fully reacted, and then polyether polyol and trimethylolpropane were further added to synthesize the nanocomposites. The mechanical property and thermal stability of the nanocomposites were investigated by wide-angle x-ray diffractormetry (WAXD), tensile test machine, TGA and SEM. The GOs evenly dispersed and no aggregation was found in PU matrix for the PU/GO nanocomposites with 0.2 % GO. However, the aggregation of GOs appeared and increased with the increasing GO content. The aggregation of GOs was harmful to the mechanical property and thermal stability of the PU/GO nanocomposites. The nanocomposites of PU/0.2% GO show the best mechanical property and thermal stability. The reduced nanocomposites of PU/GO (named as PU/rGO) were prepared by an in-situ reduction process in a solution of sodium hydrosulfite and sodium hydroxide. It follows that the GOs in the PU matrix could be partly reduced; however such partly reduction of GOs would decline the mechanical property, while enhance the thermal stability of the nanocomposites.