The molecular conformation of silk fibroin was investigated by infrared spectroscopy (IR) and circular dichroism method (CD). The silk fibroin in the liquid phase was found to have random coil structure containing a small amount of α helix. The α helix content of silk fibroin, dispersed in pure water, increased with increasing the periods during fifth instar of silkworm larvae. This fact was related to the increase of the water content in liquid silk fibroin. Silk fibroin hydrolyzed by aqueous HCl solution showed the strong IR absorption band assigned to β structure as well as the weak band at 620cm^-1 (amide V) attributing to α helix. The results of IR and CD suggest that the peptide chains consisting of α helix do not exceed several units, taking into account the x-ray diffraction pattern exhibiting no α-helical chains.