The effects of added salts on the solution properties of silk Fibroin in water were investigated by the static light scattering method. Silk Fibroin was extracted from sericin-removed yarn by 35% calcium chloride at 98°C for 6 h. Fibroin solution was obtained by dialysis of the resultant extract against distilled water at 4°C. All the experiments were done using 6% Fibroin solutions at 25°C. Monovalent salts such as NaCl and KCl induced the formation of disorderly structure via the aggregation of Fibroin molecules. On the other hand, the phase separation was observed in the presence of divalent salts such as CaCl₂ and MgCl₂. This phase separation may be attributed to the intermolecular interaction of Fibroin strengthened by the bridging effects of divalent salts. The relative viscosity of fibroin solution depended on the salts, but did not correspond to the difference in the valence of salts and the solution properties of fibroin. These results indicated that the divalent salts-induced phase separation of fibroin solution occurred in the micro-region within colloidal particles thereby not affecting viscosity strongly.