In the present study, graphene sheets were prepared via exfoliation of graphite in supercritical ethanol. Different characterization methods such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy were used to identify the initial graphite and exfoliated samples. Characterization results revealed the successful exfoliation of graphite into single to few-layer graphene. The effects of main process parameters including temperature (325-425 degrees C), pressure (20-40 MPa) and mass percent of water as a co-solvent (0-50%) on the exfoliation yield were examined by using response surface methodology (RSM) in an experimental design. The maximum exfoliation yield of 18.5% was attained at the temperature of 325.1 degrees C, pressure of 39.8 MPa and water content of 28.9 wt%, as the optimum condition. In addition, results manifest the critical role of supercritical density in the graphite exfoliation. Hansen solubility parameters (HSPs) of supercritical fluid were calculated to investigate the exfoliation of graphite in terms of supercritical density. Results indicated that modification of HSPs of supercritical fluid by tuning the supercritical condition is an effective way of improving the exfoliation yield. (C) 2015 Elsevier B.V. All rights reserved.