To investigate the volume, size, and number of microvoids in mulberry and tussah silk fibers, stannic acid gel was used as a contrasting medium to the small-angle X-ray scattering (SAXS). The influence of the stannic acid treatment on the structure of silk fibers was first investigated by using the wide-angle X-ray diffraction prior to characterization of the microvoids. The changes in crystallite size and degree of orientation with increasing stannic acid gel fraction in fibers are investigated, and it was found that the stannic acid treatment does not cause serious changes in crystallite size and degree of orientation. The changes in crystallinity indices were observed when the volume fractions of stannic acid gel in the fibers exceeded about 10%. Thus, it was confirmed that the structure of silk fibers was retained in the region of the stannic acid gel fraction less than 10%. SAXS measurements revealed that the number and the fraction of the microvoids are larger, while the sizes of the microvoids are smaller, for the mulberry silk fibers compared with the tussah silk fibers. The fraction macrovoids, however, is considered to be larger for the tussah silk fibers.