Abstract: The calculated values of refractive index and the density of paraffins, naphthenes and petroleum fractions are two factors of considerable impor-tance in hydrocarbon analysis. Though some methods have been proposed for the calculation, they are too complicated to be made useful in routine usage for hydrocarbon analysis of petroleum fractions. The authors proposed a simple, rapid and accurate method. On the basis of available data, the refractive index and density calculated by the authors method have, on the average, deviations of 0.0018 and 0.0048 grams per ml. respectively for pure saturated hydrocarbons, and of 0.0012 and 0.0018 grame per ml. for saturated petroleum fractions. Accuracy, simplicity and rapidity aro its chief advantages.