The imidization behavior and structural evolution in a microscaled film of poly(3,4'-oxydiphenylene pyromellitamic acid) precursor are studied by time-resolved synchrotron wide-angle X-ray diffraction and infrared spectroscopy to investigate the relationship between thermal imidization and structural evolution in the precursor. The precursor film displays only short-range order, but its polyimide film shows a crystalline structure based on an orthorhombic crystal lattice unit. When the precursor is heated at 2.0 degreesC/min, it undergoes imidization over the temperature range 124-310 degreesC through a two-step process: (i) decomplexation of the amide linkage from residual solvent molecules and other intra-and intermolecular amic acid groups and (ii) imide-ring closure. The maximum rate of imidization occurs at 148.4 degreesC. Anhydride rings are found to form transiently over the range 93-310 degreesC, which are attributed to the nature of the equilibrium between the precursor and its constituent anhydride- and amino-terminated species. The imidization reaction begins prior to the commencement of structural evolution. The structural evolution takes place over 132-380 degreesC as a three-step process: initiation, primary growth, and secondary growth. In particular, the initiation step requires at least 3.2% imidization. The structural evolution is further influenced by the short-range ordered structure formed in the precursor film in the process of film formation. However, the overall crystallinity in the fully imidized film is limited to only 21.4%.