Graphite flakes with sizes in the range of 3–20μm can be completely converted into single-layer graphene oxide (GO) with a high yield of 171±4% via an improved Hummers method. This yield is much higher than that (110±3%) from larger graphite flakes with sizes of 10–100μm. The structures, qualities and size distributions of the GO sheets prepared from both graphite precursors are nearly identical. More importantly, in the case of using small graphite flakes, the GO dispersion can be directly purified without the requirement of centrifugation to remove unoxidized residuals. Considering the high yield, simplified purification procedure, and high-quality of GO, the modification of using graphite flakes with small sizes is an important step towards the mass-production of GO at industrial scale.