Zeta potential is an important electrical parameter of dyeing process for textile, because it is very rapid and correct method to investigate dyeing phenomena. In order to investigate the surface dyeability and total adsorbed dye amounts of silk fiber with acid dye, zeta potential was measured by streaming dye solution of varying concentrations and electrolyte through the silk fiber, then, after desorption of dye from dyed fiber, total dye amounts was measured by Lambert-Beer method. As the results, 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. Then the zeta potential approaches saturated value. These results were explained on the basis that the dye anions with their negative charge makes salts with NH3+ groups of silk fiber and the formation of hydrogen bond between silk fiber and dye. The surface area of silk fiber covered with the adsorbed dye molecules which was calculated from the reciprocal of slope of relation between the surface dye adsorption and total dye adsorption was much higher than that of nylon fiber.