An ab initio (HF/6-31G) study of the molecular structure of polychlorinated
biphenyls (PCBs) has been carried out using Gaussian 98W. The molecular geometries of PCBs28、PCBs52、PCBs101、PCBs103、PCBs118、PCBs138、PCBs153、PCBs180 and PCBs204 were optimized using analytical gradients optimization routines. According to the bond length, bond angle, dihedral angel, energy value and dipole moments obtained from the calculation, the structure character of PCBs congeners and the relationship between the molecular structure and toxicity have been discussed. The theoretical calculations were in good agreement with the experimental results. And the structure parameters gained from our calculation are a useful reference for studying the interaction between the PCBs congeners and the environment.