Recently, an interest from both academia and industry has been moved toward ionic liquids due to their environmentally friendly characteristics as green alternative for traditional volatile organic solvents (VOCs). In addition, because of their wide range of physicochemical properties they found unique applications in renewable energy sector. For using such substances, one needs reliable correlations for predicting their physical properties. In the present work, a new method for determining the density of ionic liquids has been proposed. It has been shown that density of ionic liquids appears to be correlated linearly with the refractive index parameter of these liquids. An average error of 12.27% for 45 ionic liquids was obtained.Several equations of state were compared with this method including, SRK, RK, Peng Robinson and Riazi and Roomi [1].Using a sample of 29 ionic liquids, average errors of 88.72%, 65.79%,39.10% and 66.07% were obtained for these equations of state, respectively. The estimated values from this method seems to be reasonable and can be applied to determine the density of ionic liquids at any temperature.