In order to estimate the degree of aging of Liao Dynasty silk, excavated from the Faku Yemaotai Tomb in Liaoning, sodium dodecyl sulfate – polyacrylamide gel electrophoresis (SDS-PAGE) was adopted to analyze the relative molecular weights of silk proteins. The results were compared with modern silk protein to explore the aging characteristics of the Liao Dynasty silk. The SDS-PAGE results showed that the Liao Dynasty silk protein had two peptid chains, 65kD and 56kD. The 25kD light chain, which usually exists in silk protein, was not found. The protein structures of the Liao Dynasty silk show significant changes, indicating serious aging. Further analysis found that the two peptide chains of 65kD and 56kD were the fragmentation products of the silk protein heavy chain (390kD), while the disappearance of the 25kD protein suggested that the peptide chains in the non-crystalline region had broken down, or had been degraded into smaller peptide chains. X-ray diffraction analysis showed that the non-crystalline region peptides degraded much faster than did the crystalline region peptides. Therefore, the degree of crystallization of Liao Dynasty silk, which had been buried for many years, is higher than that of modern silk.