The contents of diferent structural deftects of five PVC samples，obtained by polymeri-zation at monomer unsaturation vapor pressure，and five commercial PVC samples were deter-mined．The total number of double bonds(C=CT) and labile chlorine atoms (ClL) were deter mined by bromine addition and phenolysis methods，respectively．The determination of inter-nal double bonds (C=Cint) and isolated doubl bonds(C=Ciso) was carried out by means of ozonlysis and FTIR methods．It was found tha：the dehvdrochlorination rates of PVC samples depended linearly on the contentof ClL，C=CT and (C=C)lso，respectively．In addition，the con-tents of these three structural defects were all correlated linearly with 1／Mn，so most parts of them would be the same defect——terminal allyl chlorine．Therefore，the main source of thermal dehydrochlorination of PVC could be attributed to the presence of terminal allyl chlorine groups．A mechanism．that in the presence of HCl as eatalyst the terminal allyl chlorine can be isomerized into internal allyl chlorine．was proposed to explain the main role of terminal allyl chlorine in the thermal dehvdrochlorination of PVC．