The elastic modulus E-l of the crystalline regions in the direction parallel to the chain axis was measured by X-ray diffraction for polyimide derived from polyamic acid (biphthalic dianhydride and p-phenylene diamine). These specimens were cured by two different routes: curing at 200 degrees C, and at 400 degrees C for 1 h, respectively (2STEPS), and curing from 80 degrees C to 400 degrees C stepwise (nine steps) for 1 h at each step (STEPWISE). The E-l values of 54-169 GPa were obtained for the STEPWISE specimen and 80-178 GPa for the 2STEPS specimen, depending on the meridional reflection employed for measurement of the E-l value. A linear relationship between the E-l value and the fiber identity period was obtained from each meridional diffraction, such that the E-l value increased with an increase in the fiber identity period. The E-l value of the fully extended structure was estimated to be 210 GPa. These are considered to be due to the coexistence of polymorphs with different skeletal structures. The crystalline regions of the 2STEPS specimen seems to consist of a more extended skeleton than those of the STEPWISE specimen.