Mulberry silk plain Habutae and Antheraea yamamai silk yarn immersed in distilled water and NaOH solution (pH=13.0±0.2) for 48 hours were folded by the use of M.I.T Paper Folding Tester, and then their properties of tensile strength and elongation were investigated. By folding in dry conditions the tensile strength and elongation of plain Habutae and Antheraea yamamai were both decreased on a large scale. On the other hand, in wet conditions these values were decreaseda little. Especially in the case of warp yarn of plain Habutae and Antheraea yamamai twisted softly, it was recognized that the tensile strength and elongation were increased by folding in wet conditions.
It was found that folding endurance in wet conditions was related closely to the twisted state of yarns.