C¹³-NMR spectroscopic method could be very successfully applied to study the microstructure of the chlorinated IPP and to compare the relative reactivity of the three kinds of protons toward chlorination. The change in thermal properties of the chlorinated IPP''s as a function of degree of chlorination was also investigated by DSC. In the chlorination of IPP the relative reactivity of the three different kinds of protons contained in the structure unit of IPP, namely methyl, methylene and methine protons, could be quantitatively determined, decreasing in the order, methylene >methine >methyl. The chlorine atoms were found to be quite uniformly distributed along the polymer chain. The glass transition temperature (T_g) of the chlorinated IPP''s increased gradually with increase of chlorine content, but with increase of the chlorine content the melting point (T_m) was lowered rapidly and crystallinity was no longer observed when the chlorine contents reached 25%.