In this paper, the monomers 4,4'-oxydiphthalic anhydride (ODPA), 3,3', 4,4'-biphenyl dianhydride (BPDA), 3;4'-oxidianiline (3,4'-ODA), and 4,4'-oxidianiline (4,4'-ODA) were selected to synthesize polyimides (PI) or copolyimides (co-PI) in N,N-dimethyl acetamide (DMAc) by two-step method. To control the PI molecular weight (MW), phthalic anhydride (PA) was used as the end-capping reagent. The effect of far-infrared radiation (FIR) on PI imidization was investigated by Fourier transform infrared spectroscopy (FTIR). Some factors affecting imidization process in FIR were discussed, including PI imidization time and temperature, molecular structure, designed number average molecular weight, crystalline, imidization procedure, film thickness, polyamide acid (PAA) solid content, and so forth. The PI imidization process in FIR will be affected by all these factors. (c) 2005 Wiley Periodicals, Inc.