Large-sized graphene oxide (LGO) is desirable for many graphene-based applications. Herein, for the first time, we synthesized LGO (83 mu m average size) using electrochemically modified graphite as the precursor. With this strategy, we could reduce both the oxidation time (1 h) and oxidant dosage (2 equiv.) while achieving a fast and high-yield (similar to 90.3 wt%) LGO preparation. LGO papers show much better mechanical properties and nearly four times higher conductivity (after reduction) than their counterparts assembled using small-sized GO. The results demonstrated that electrochemical pre-treatment is an efficient approach to accelerate the oxidation of graphite at low oxidant dosages and to control the size of the resulting GO. (C) 2019 Elsevier Inc. All rights reserved.