The crosslinking reaction of bicyclo[2．2．1]hept-5-ene-2，3-dicarboxylic anhydride capped polyimide by using N-phenyl-bicyclo[2．2．1]hept-5-ene-dicarboximide (I) as the model compound was studied．It is found from the changes of the conversion and the molecular weight of the polymers with reaction temperature and time that endo-I could be polymerized above 275℃ and the polymerization tended to the completion at 800℃ for 2hr．NMR spectra of endo-I polymers show that the following structural units were present in the pulymer：The contents of different types of double bonds in the polymer could be determined semi-quantitatively by NMR spectroscopy and the bromine addition method．The ex- perimental results indicated that the average molecular weights of the polymer are 800一1000 and the total contents of double bonds are 1.00一1.25 per molecule．while the con-tents of double bonds of cyclopentene type are smaller than 0.5． Under the same reac-tion conditions the structure of the polymers from endo．and exo-I is similar．As a result，the following mechanism of polymerization was proposed．The retro Diels-Alder reaction took place at first on heating I．The active centres were produced above 275℃．then the chain growth reaction was followed．The trasformation of confi-guration．rearrangement and different paths of chain growth reactiOIlS occurred during the polymerization．The chain termination was caused mainly by disproportion reactions．