In this work, a new technique is introduced for characterizing C6+ fractions using an equation of state (EOS). This approach uses readily available properties of C6+ fractions (density, molecular weight and normal boiling point) to select model molecules representing these fractions, which may then be used for predicting phase behavior. The chain of rotators group contribution EOS (CORGC EOS) is used in this work for characterizing the C6+ fractions. The validity of the proposed technique is checked by comparing predictions with 15 sets of experimental literature data, including eight synthetic C6+ mixtures and seven real petroleum fractions. Phase behavior predictions of the eight synthetic mixtures showed an total average absolute percent deviation (AAD%) of 2.33% when the known compositions were used, while a total AAD% of 3.58% resulted when these synthetic mixtures were characterized using the proposed method. The characterizing method was also compared with. the Aasberg-Petersen and Stenby method for phase behavior predictions of a real natural gas-crude oil mixture. Results showed that this method gives better results for liquid phase compositions (total absolute error = 0.7%), while the Aasberg-Petersen and Stenby method gives better results for vapor phase compositions (total absolute error = 2.36%).