An investigation was carried out on the effects of annealing treatment on the molecular structure and the mechanical properties of isotactic polypropylene fibers annealed in an air heated environment at temperatures ranging from 60 to 140 degrees C. Analysis of the equatorial X-ray diffraction traces showed the presence of a three phase system of amorphous-smectic-monoclinic forms and revealed the transformation of the metastable smectic form to the highly stable monoclinic form as the annealing temperature is increased, resulting in an enhanced degree of crystallinity and the crystallite size. The improvements in the degree of crystallinity and the crystallite size became more remarkable above 120 degrees C. Evaluation of the crystallinity was carried out using an analysis of density, infrared spectroscopy, and X-ray diffraction methods whereas the state of the molecular orientation was evaluated using polarized infrared spectroscopy measurements only. Polarized infra-red spectroscopy measurements after the curve fitting procedure showed a slight increase of the molecular orientation of the helical chain segments present in the crystalline phase represented by the IR bands at 841 and 998 cm(-1) whereas the amorphous structure represented by the IR band at 974 cm(-1) showed no significant change with increasing annealing temperature. The improvement in the molecular orientation of the crystalline phase became more remarkable above 120 degrees C. Tensile strength of the annealed fibers increased with increasing annealing temperature but the elongation at break and the initial modulus were not affected as much as the tensile strength. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 3322-3338, 2011