A new empirical approach has been developed to estimate parameters of the modified Group Contribution Perturbed-Chain statistical associating fluid theory (mg-SAFT) for alkane and alkene isomers. A key advantage of this approach is that the molecular parameter of isomer components is distinguished simply through the chain length parameter. New group parameters for 1-alkenes, cis- and trans-allcenes, methyl-alkanes and dimethyl-alkanes are determined. Several pure compound properties such as the vapor pressure, liquid density, enthalpy of vaporization, heat capacities are predicted. It has been found that the use of two adjustable parameters in determining the chain length parameters of different isomer groups in the fitting procedure for alkane or alkene isomers is enough to provide a good description of PVT and some derivative properties. Test and comparison with other existing SAFT-based group contribution approaches were performed in calculating the vapor pressure and saturated liquid density of heavy compounds. The results show that, mg-SAFT describes the fluid phase behavior for light compounds with a comparable accuracy compared to other group contribution approaches and much better for heavier compounds (prediction results). (C) 2016 Elsevier B.V. All rights reserved.