Polyethersulfone (PES) hollow fiber membranes were prepared by both wet spinning technique, without gas-gap, and dry/wet spinning technique under different gas-gaps, namely, air, oxygen, nitrogen, carbon dioxide, argon and humid air. All spinning parameters were maintained the same except the type of the gas-gap used. Flat-sheets were prepared by the phase-inversion method using the same polymer solution and water as coagulant. The X-ray diffraction pattern of virgin PES indicated that this polymer is mainly amorphous in nature and X-ray diffraction study revealed quite similar structure for flat-sheets and hollow fibers prepared from the PES polymer. It was observed that the PES hollow fibers prepared with dry/wet spinning technique exhibited some differences in X-ray diffraction spectra showing attenuation of the broad band peak intensity for the hollow fibers prepared under argon and carbon dioxide. The intersegmental distance of polymer chains, d-space, was determined. A value of about 4.77 +/- 0.05 angstrom was obtained. The results provide evidence that the spinning technique do not induce crystallization of hollow fibers.