Statistical associating fluid theory (SAFT) is modified with two improvements.One improvement is the consideration of the segment shape of chain molecules in the reference term with the Hard Convex Body equation of state.The other is the change of dispersion term because of the chain formation.The first improvement makes better expression of critical points for small non-sphere molecules and the second for chain molecules.With these two modifications

the theoretical critical compressibility factor of the extended SAFT is in the range of 0.23-0.31

that is the same as those of such real nonpolar fluids as alkanes.The extended SAFT is used to correlate and predict saturated pressures and saturated liquid densities with the average relative deviations less than 1% for most of n -alkanes and branched alkanes.Meanwhile the predicted critical temperatures

pressures and densities are in good agreement with experimental ones.This equation of state can be used in thermodynamic calculation for systems with components in the critical region.