The crystalline fraction of silk fibroin (Fcp) was obtained from the aqueous solution of silk fibroin hydrolyzed by $alpha$-chymotrypsin. The molecular weight of Fcp was found approximately 7000 by using high speed GPC. On the other hand, a high molecular weight of PIFcp product could be obtained by the reverse reaction of enzymatic proteolysis of Fcp precipitates. Some parts of this PIFcp have the molecular weights of approximately 17000 and 24000. As a result of x-ray diffraction analysis, the crystal structure of Fcp and PIFcp was turned out silk-II type and silk-I type, respectively. Upon the reverse reaction of enzymatic protelysis, the structural transition occured from silk-II type to silk-I type crystal for the most of Fcp precipitates. It was confirmed that PIFcp might be somewhat stable crystal structure of silk-I type according to the thermal analysis as well as x-ray diffraction method.