The purpose of this article is to study the effects of mechanical properties on seam shrinkage of lightweight silk fabrics. Thirty five silk fabrics are used for experiment. Mechanical properties of the fabrics on warp and weft directions were tested by FAST instrument. Seam shrinkage rate were tested. Curve regressions were used to analyze the influence of mechanical properties on seam shrinkage. The results showed that the bending rigidity and extensibility were closely correlated with the seam shrinkage rate, while the formability and rigidity were not the main factor of affecting the seam shrinkage of the light-weight silk fabric. The regression equations of bending rigidity and seam shrinkage rate, extensibility and seam shrinkage rate were obtained.