The thermodynamic properties of 76 polychlorinated dihydrophezines (PCDPs) in the gaseous state at 298.15 K and 101.325 kPa, have been calculated using the density functional theory (the BHANDHLYP/6-31G*) with Gaussian 03 program. Based on these data, the isodesmic reactions were designed to calculate the standard formation heat (Delta H-f(theta)), standard Gibbs free energy of formation (Delta(f)G(theta)) of PCDPs in the gaseous state. The relations of these thermodynamic parameters with the number and position of chlorine substituents (N-PCS) were discussed, and it was found that there exist good correlation between thermodynamic parameters, including heat capacity at constant volume (C-V(theta)), entropy (S-theta), enthalpy (H-theta), free energy (G(theta)), Delta H-f(theta), Delta(f)G(theta), and N-PCS. The relative stability order of PCDP congeners was theoretically proposed based on the relative magnitude of their Delta(f)G(theta). In addition, the values of molar heat capacity at constant pressure (C-p,C-m) for PCDP congeners have been calculated.