For the purpose of practical applciation of methylation of silk fibroin fibers by diazomethane for the improvement of its light-resistance, diazomethane treatments in acetone medium were investigated at various conditions, in place of ether medium which is customarily used.1. As the reaction medium, acetone is more effective for the higher degree of methylation than ether on account of its high boiling point.2. Degree of methyltion is considerably affected by the moisture content of silk fiber before the treatment. Optimum moisture content for effective methylation is about 4% of silk.3. External apperance and mechnical properties of silk are not at all changed by the methylation.4. By the higher methylation (about 3% methoxyl content), the lightresistance of silk fiber is remarkably improved.5. From the data of reaction rate and moisture absorption capacity of methylated silk, some considerations are carried on in relation to the accessibility of silk fibroin fiber.