The structural and thermodynamic properties of 75 polychlorinated thianthrenes(PCTAs) in the ideal gas state at 298.15 K and 1.013 ×105 Pa have been calculated at the B3LYP/6-31G level using Gaussian 98 program.Based on the output data of Gaussian=the isodesmic reactions were designed to calculate standard enthalpy of formation(△fHθ) and standard free energy of formation(△fGθ) of PCTAs congeners.The relations of these thermodynamic parameters with the number and position of Cl atom substitution(NPCS) were discussed=and it was found that there exists high correlation between thermodynamic parameters(total energy(TE)=zero-point vibrational energy(ZPE)=thermal correction to energy(Eth)=heat capacity at constant volume(Cvθ)=entropy(Sθ)=enthalpy(Hθ)=free energy(Gθ)=standard enthalpies of formation(△fHθ) and standard Gibbs energies of formation(△fGθ)) and NPCS.On the basis of the relative magnitude of their △fGθ=the order of relative stability of PCTA congeners was theoretically proposed.In addition=the correlations between structural parameters and NPCS were also discussed.The good correlations were found between molecular average polarizability(α)=energy of the highest occupied molecular orbital(EHOMO)=molecular volume(Vm) and NPCS=and all R2 values are larger than 0.95.Moreover=it was supposed that the isomer groups with higher toxicity should be Tri-CTA and TCTA . The structural and thermodynamic properties of 75 polychlorinated thianthrenes (PCTAs) in the ideal gas state at 298.15 K and 1.013 × 10 5 Pa have been calculated at the B3LYP / 6-31G level using Gaussian 98 program. Based on the output data of Gaussian = the isodesmic reactions were designed to calculate standard enthalpy of formation (ΔfHθ) and standard free energy of formation (ΔfGθ) of PCTAs congeners. These relations of these thermodynamic parameters with the number and position of Cl atom substitution (NPCS) were discussed = and it was found that there exists high correlation between thermodynamic parameters (total energy (TE) = zero-point vibrational energy (ZPE) = thermal correction to energy (Eth) = heat capacity at constant volume (Cvθ) = entropy (Sθ) = enthalpy (Hθ) = free energy (Gθ) = standard enthalpies of formation (ΔfHθ) and standard Gibbs energies of formation (ΔfGθ)) and NPCS.On the basis of the relative magnitude of their ΔfGθ = the order of relative stability of PCTA congeners was theore tically proposed. In addition = the correlations between structural parameters and NPCS were also discussed. The good correlations were found between molecular average polarizability (α) = energy of the highest occupied molecular orbital (EHOMO) = molecular volume (Vm) and NPCS = and all R2 values ​​are larger than 0.95. Moreover = it was supposed that the isomer groups with higher toxicity should be Tri-CTA and TCTA.