Semicrystalline structures involving molecular packing and optimized chain conformations of three polyimides have been obtained with the charmm molecular-dynamics program, and their densities calculated. An eclipsed conformation of face-to-face stacking of polymer chains with a relative shift is observed, and the shift is found to depend upon the molecular characteristics of the planar unit and chain linearity. Three-dimensional structures obtained by calculating the optimized edge-on confirmation of the stacked units yield the smallest calculated density for PMDA-ODA, namely 1.515 g/cm3, followed by BPDA-PDA with 1.644 g/cm3 and PMDA-PDA with 1.712 g/cm3. Both PMDA-ODA and BPDA-PDA reveal planar zigzag structure, and the former shows the largest undulation measured by the maximum difference in surface elevation on the surface cleaved along the edge-on bonds. PMDA-PDA reveals flat planar structure.