The evolution of chemical composition and structure during the thermal imidization of an ester-type polyimide precursor, poly(4,4'-oxydiphenylene p-pyromellitamic diethyl ester), in micrometer scale films were studied for a heating rate of 2.0 degrees C/n-Lin with time-resolved synchrotron X-ray diffraction, in-situ infrared spectroscopy, and modulated differential scanning calorimetry. Our analyses show that the precursor polymer undergoes imidization in a two-step process. In the first step, the precursor polymer is decomplexed from the residual solvent molecules, and in the second step, it undergoes imide ring formation with the, release of ethanol as a byproduct. The imidization reaction starts around 210 degrees C and continues up to 320 degrees C. The thermal imidization reaction induces the structural evolution of the film. As the imidization reaction proceeds, the coherent length along the polymer chain axis increases. This imidization-induced structural evolution was found to occur via three steps: (i) initiation, (ii) the first crystallization, and (iii) the second crystallization. The initiation step is necessary prior to the evolution of the crystalline structure to increase the chain mobility of the precursor polymer chains, and it requires thermal heating up to at least 238 degrees C at which point 22.5% of the imidization is complete. Thereafter, the first crystallization occurs up to 310 degrees C, at which point 98.3% of the imidization is complete. In the range 310-380 degrees C, the second crystallization occurs and produces almost complete imidization of the polymer chains.