Thermal imidization of the polyamic acid derived from BPADA and BAPS was investigated mainly by GPC and FTIR equipped with an in-situ hot cell．The non-isothermal imidization was carried out in the temperature range from 100℃ to 300℃ with a multi-step heating program．Resalts indicated an initial dissociation of the amide bonds,producing anhydride and amine end groups,and the reduction in molecular weight and the embrittlement of the film．As the temperature was increased further,the above terminal functional groups recombined to increase the chain length and molecular weight．resulting in the toughness recovery of the film．The isothermal imidization included two steps of different reaction rates,afast stepfollowed by a slow one．Thatisto say a kineticinterrupttook place．A two-step first-order kinetics model was applied to the isothermal imidization,and the relevant parameters,such as k．E．and A,were obtained In addition,as the isothermal imidization was concemed,a new empirical formula was pmposed to correlate the degree of imidization at the kinetic interrupt point with the curing temperature,which sbowed a good agreement with the experimental date Moreover,the kinetic interrupt was explained based on both the conformation change and the effeet of free volume.