main subunits of the silk fibroin were separated by Sephadex G 200 when the silk fibroin was reduced by 2 mercaptoethanol. Then the amino acid compositions of the native silk fibroin and a-、c- subunits were measured and analyzed. The result about silk fibroin subunit composition discussed in the former paper was tested and verified. And some properties of silk was interpreted in view of microcosmic structure of silk fibroin.