Four kinds of silk fibers. three mulberry silks from Korea, China and Brasil and one Chinese Tussah silk have been hydrolyzed in 5% HCI solution at 95 $pm$1$^{circ}C$ at different time intervals. The change of weight loss, hydrolysis rates, and crystallinity of the silt fibers was measured. Crystallize size, X-ray order factor, and IR crystallinity index have been determined fur the control and 3 hour-hydronbroins. The results arc discussed in connection with the fine structure of the crystalline region of the silk fibroin. The conclusions obtained from this study are as follows: 1. The hydrolysis rate of the silk fibroin in the 5% HCI solution has been changed at three stages, and hydrolysis rate constant(t) was in inverse proportion to the amount of the crystalline region. The three-stage hydrolysis was explained on the basis of the three-phase structure of the silk fiber. 2. As the silk fibroin changed to the hydrofibroin by HCI treatment for 3 hours, the crystallite size increased. 3. t in the crystalline region was in inverse proportion to the crystallite size. 4. The order factor the crystallinity index, and total crystallinity indicated different values in connection with degree of crystallinity. 5. The crystallinity of the mulberry silk was greater than that of the Tussah silk.