The heterogeneous structures of poly(glycolic acid) (PGA) fibers which have been used as bio-degradable suture were studied by differential scanning calorimeter (DSC), X-ray diffraction and C-13 solid state NMR. The C-13 cross polarization NMR spectra without magic angle spinning of the stretched fibers observed by changing the angle between the fiber axis and the magnetic field clearly showed the heterogeneous structures which consist of three components; well-oriented, poorly-oriented and isotropic amorphous components. The local structure, distribution of the fiber axis and fraction of each component were determined quantitatively. Change in the heterogeneous structure by changing the stretching method in the sample preparation and by changing the stretching ratio was also monitored. The X-ray diffraction data of the fibers are in good agreement with the C-13 CP NMR data. Change in the heterogeneous structures correlate with change in the thermal properties observed by DSC method. The molecular dynamic simulation showed the generation of trans conformation of PGA chain and also change in the fraction of other conformations by stretching, which supports the experimental results obtained above and gives additional structural information. (C) 2009 Elsevier Ltd. All rights reserved.