The structural characterization of chiral nylon-3 derivative, poly (3-ethyl-3-phenyl-2- azetidinone) (PEPA) has been carried out using FTIR, thermal analysis, X-ray diffraction and electron microscopy. X-ray diffraction of power and fiber samples were analyzed together with electron diffraction patterns of single crystals obtained from isothermal crystallization in solution. Two different crystalline forms were found in accordance with DSC traces, FTIR spectra and X-ray diffraction diagrams. Electron microscopy corroborates the morphological and crystallographic differences between both crystalline forms. Molecular modeling was applied to characterize the two crystalline forms, which were related to different conformation and consequent space symmetry into the crystal unit cell