The present study was aimed to clarify the electrokinetic phenomena of dyeing through the role of electric double layer by ion in silk fiber with different ionic effects under different condition. Spectrophotometric analysis was used to compare dyeing condition of surface, which can be detected by electrokinetic phenomena and the inner of fiber after decoloration of dyed silk fiber. Silk, the typical amphoteric fiber was dyed with C.I.Acid Orange 20 and C.I. Acid Orange 10. Various combinations were prepared by combining pH, temperature and dye concentration, in order to generate streaming electric potential which were measured by microvoltmeter and conductivity meter. The results were transformed to zeta potential by Helmholtz-Smoluchowski formula and to surface electric charge density by Suzawa formula. The amount of surface dye, the amount of dyes on inner fiber, the effective surface area of fiber, the degree of swelling, and the amount of end group on fiber were also measured and discussed. The main results obtained are as follows. 1. The zeta potential was proportional to the amounts of dye adsorbed on silk fiber and the negative zeta potential increased with the amounts of dye adsorbed. 2. The isoelectric points of fibroin and sericin were different. 3. The degree of swelling of silk was closely related to zeta potential and the negative charge of heat treated silk increased in accordance with the rising temperature to 150$^{circ}C$. 4. The effective surface area of silk fiber for acid dyeing was about 1.1 ${ imes}$107$ extrm{cm}^2$/g-fiber.