When exposed to ultraviolet rays of wavelength 360 nm (visible rays were filtered), cocoons fluoresced colors such as yellow. Yellow fluorescent cocoons have higher reelability percentages than violet fluorescent cocoons. A cocoon lot was artificially divided into high-reelability percentage lots and low-reelability percentage sub-lots. The cocoon fluorescent color distribution of the respective sub-lots was observed and solubility of cocoon layer sericin was measured, and both were compared. In the results, reelability percentage could not be determined by fluroescent color distribution in the case of cocoon lots of the various mixed fluroescent color cocoons. On the other hand, cocoon layer sericin separated by fluorescent color was observed. Yellow fluorescent sericin was more soluble than violet in boiling water. When treated with boiling water, this yellow substance was rapidly dissolved from silk sericin. As for adheison of cocoon filaments, which causes problems during the reeling process, the soluble yellow substance was favourable for separation of adhesive cocoon filaments. Further, it was considered that violet fluorescent cocoon sericin was slow in separation of cocoon filaments and worse than yellow fluorescent cocoon sericin. Based on the above reason, the author proposes a possible reeling method to improve reelability in which a cocoon lot is grouped into sub-lots depending on fluorescent color observed under ultraviolet rays.