In this study, commercial polyurethane (PU) and graphene nanoplatelets (GnP) and their derivative graphene oxide (GO) were used to fabricate PU composites. The size effect of fillers on mechanical properties and anti-corrosion performance of as-prepared composites was thoroughly investigated. It was found that GO was more uniformly dispersed in the PU matrix than GnP due to its compatibility with PU. Furthermore, GO led to the higher mechanical properties and anti-corrosion performance than PU/GnP composites, and the properties were strongly dependant on the size of the GO. Specifically, incorporating large sized GO (GO-M25) in 0.5 wt% indicated the highest average synergetic tensile modulus up to 53% from the neat PU and the lowest corrosion rate of 0.001 MPY (1 MPY = 0.547 g center dot m(-2) center dot d(-1)). This phenomenon was attributed to the fact that the larger size of GO is not only uniformly dispersed within the PU matrix but also enables interaction between PU and GO. Conversely, PU composites incorporated with the small sized GO (GO-C750) did not show elastic behaviour from 0.1 to 0.5 wt% of the filler. This is due to the fact that the high surface area and hydrophilic functionalities of GO-C750 resulted in hard-segment content reduction in PU. This research can help in the design of a PU coating that is physically improved and that has a superior anti-corrosive capacity, particularly for pipelines in the oil-sands transportation industry.