As far as the sericin fractions are concerned, two contradictory opinions have been suggested, that is, the one is that they are different protein and the other identical protein. It is evident that no satisfactory conclusion concerning them in many investigations could be reached. In present study, an attempt was made to throw light on the questions about two opinions and the existence of sericin fraction. From the results of amino acid contents were identical. and so they are considered to be a similar compound. It was also observed that the dissolution rate of sericin according to approaching to the inner layer was decreased. This shows that the layers of sericin surrounding fibroin are gradually formed with increasing molecular weight of sericin according to approaching to the inner layer. On the other hand, the increase in amino nitrogen contents of sericin solution with increased time of autoclaving indicates that the hydrolysis of sericin molecules occurs in the extracting process. On the account of the above two phenomena, the sericin molecules in extratcted sericin solution are diversified with different kinds of molecular weight, and so sericin fractions are separated.