The powders of Bombyx mori silk fibroin were prepared from aqueous solutions of silk fibroin by several kinds of the insolubilization methods, some of which used methanol. The insolubilization appeared to be effected by the conformational transition of silk fibroin from the random coil to antiparallel β-sheet form, which was monitored with IR spectroscopy. In order to characterize the structure of the power in the swollen state in water, a spin-labeled silk fibroin powder was prepared and the ESR spectra were observed, as well as 13C-NMR. The heterogeneous structure was analyzed quantitatively in terms of the fraction of fast, slow, and very slow components in the ESR spectra. Finally, an enzyme, invertase, was immobilized with the silk fibroin powder. The thermal stability of the enzyme was much improved by the immobilization.