Density functional theory (DFT) andab initiomethods were used to optimize the molecular geometry of 2,3,7,8-TCDD at B3LYP/6-31G, B3LYP/6-31G*, B3LYP/6311G* and MP2/6-31G* levels, respectively. Harmonic vibrational spectrums have been performed at B3LYP/6-31G* and B3LYP/6-311G* levels. The standard thermodynamic parameters such as heats of formation, standard entropy, standard enthalpy, heat capacity at various temperatures have been evaluated using the scaled B3LYP/6-31G* and B3LYP/6-311G* frequencies. Semi-empirical molecular orbital method with the PM3 Hamiltonian at restricted Hartree-Fock level (RHF) was also performed to compare the geometrical optimization and thermodynamic properties with those methods mentioned above. The results show that: (1) 2,3,7,8-tetrachlorinated dibenzo-p-dioxin(TCDD) is a planar molecule, optimal geometries obtained by B3LYP/6-31G, B3LYP/6-31G*, B3LYP/6311G * andMP2 / 6 - 3 1 G* arealmostthesame, thelargestdifferenceinbondlengthbetweenthemis 7.9 pm, the largest difference in bond angle is 0.55o, and the largest difference in rotational constant is 0.03 GHz; (2) There are 60 vibrational frequencies for this molecule, of which there are 10 strongest intensities of vibration bands. The computed frequency scaled by 0.9628 for B3LYP/6-31G* and scaled by 0.9711 for B3LYP/6-311G* have similar frequencies and intensities of vibration bands, both are in good accordance with experimental values. (3) The thermody namic parameters such as standard entropy, standard enthalpy, heat capacity, heats of formation at temperatures from 298~1500K have been calculated, All the thermodynamic parameters increase with the increment of temperature, and the difference between B3LYP/6-31G* and B3LYP/6-311G* is small, PM3 method alsopredictsgood results, and ingood accordancewithother′sworksandexperimen talvalues. (4) The optimal geometry and thermodynamic parameters of 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (TCDD) obtained in this paper would be fundamental databases for further research on generation, decomposition and kinetics study of dioxins.