This study deals with the analysis of structure, physical properties, and dyeing behavior of silk fibers with different filament sizes. Fine and coarse silk fibers were obtained from Akebono and Ariake cocoon varieties, respectively. Both samples exhibited a fairly similar x-ray crystallinity, while the degree of molecular orientation increased with decreasing the fiber size. Tensile strength and energy of fine silk fibers were significantly higher, while elongation at break did not change in relation to the fiber size. Fine silk fibers exhibited a slightly higher thermal stability, as shown by the upward shift of both the DSC decomposition temperature and the TMA final extension step at above 300 degrees C. The TGA and DMA (E ") patterns remained unchanged regardless of fiber size. The amino acid analysis confirmed the absence of any difference of chemical structure between fine and coarse silk fibers, the content of acidic, basic, and other characteristic amino acid residues being exactly the same. Accordingly, both samples adsorbed the same amount of hydrochloric acid. Exhaustion dyeing tests with various direct, acid, and reactive dyes showed the occurrence of appreciable differences in the perceived color between fine and coarse silk fibers. The size of color difference was quantitatively evaluated by reflectance measurements and discussed in relation to silk fiber morphology, structure, as well as dyeing conditions. The kinetics of diffusion of two model dyes was not significantly affected by the different fiber size and structure.