Abstract: 1. A simple method is presented for determining double bond index of pure unsaturated hydrocarbons, petroleum fractions and other hydrocarbon mixtures based on a correlation of observed refractive index, calculated refrac-tive index, molecular weight and the number of double bonds per molecule. The data neccessary are: (a) mid-boiling point, (b) refractive index at 20℃ for sodium D line, (c) density at 20℃ and (d) weight percent hydrogen of the sample. 2. The double bond index determined by the present method for pure unsaturated hydrocarbons and unsaturated concentrate from petroleum fractions agree fairly well with that determined by Lipkin, Sankin, and Martins method, but there is some discrepancy for petroleum fractions containing saturated hy-drocarbons. 3. For identifying the predominant type of aromatic or olefinic hydrocar-bons in a petroleum fraction, the double bond index should be determined on unsaturated concentrate when the present method is used.