High-pressure annealing in oxygen ambient at low temperatures (∼300 °C) was effective in improving the performance of graphene field-effect transistors. The field-effect mobility was improved by 45% and 83% for holes and electrons, respectively. The improvement in the quality of Al2O3 and the reduction in oxygen-related charge generation at the Al2O3-graphene interface, are suggested as the reasons for this improvement. This process can be useful for the commercial implementation of graphene-based electronic devices.