The small angle X-ray scattering of domestic and wild silk fibers were examined under air-dried, water-swollen, and sodium hydroxide aqueous solution-swollen conditions. The scattering intensity of those fibers were higher in water-swollen state than in dried state. It became much higher when a sodium hydroxide solution was used to swell the specimens, exhibiting a maximum or a shoulder in the scattering curve. This suggests the appearance of structures having a long period in the direction perpendicular to the fiber axis. The increase of scattering intensity was larger in domestic silk fiber than in wild silk fiber. The weight-loss of fibers due to the treatment with a sodium hydroxide solution was also larger in domestic silk fiber. The increase in scattering intensity of swollen domestic silk fiber became less marked when they had been subjected to the treatment with a metal-salt solution or the tannin fixing treatment in advance. These experimental evidences suggest that the molecular chains in amorphous region of the wild silk fibers seem to be rigidly linked by the coloring and the mineral matter.