Graphene nanomesh was prepared via one-step hydrothermal process in a teflon-lined autoclave. The graphite flakes were used as the raw materials and no templates were adopted during the preparation. Experiment results showed that as the hydrothermal temperature increased, there was an obvious transition in the structure and morphology of the final products. When the temperature was set at 95 A degrees C, the graphite was oxidized to graphite oxide. The structural transition of the sample could be observed at 115 A degrees C, part of the sample was reduced during the hydrothermal treatment. When the temperature reached to 135 or 155 A degrees C, there appeared signals related to graphene; no obvious information about graphene oxide could be detected. Some holes were discovered in the samples. These structural changes were confirmed by X-ray diffraction, thermo-gravimetric analysis, and Fourier transform infrared spectroscopy and laser Raman spectroscopy. Moreover, atomic force microscope and transmission electron microscope revealed the as-prepared graphene was monolayer or few-layer nanomesh.