In order to investigate the surface dyeability of the nylon 6 fiber in acid dye solution, the ζ-potential, the surface dye adsorption and the total dye adsorption of the nylon 6 fiber (undrawn) in an acidic solution (pH 4.0) of monoazo acid dyes - Orange II, I and IV were determined. The isoelectric point of the fiber was pH 4.58. With increasing dye concentrations, the sign of the ζ-potential of the fiber in dye solution changes from positive to negative because of the electrostatic bonds between the NH3+ of the fiber and the dye anion, RSO3-. Then the ζ-potential approaches the saturated value. Both, the surface dye adsorption and the total dye adsortion and the total dye adsorption increase with increasing dye concentrations. These dye adsorptions became smaller in the order of Orange IV, I and II. This fact may be attributed to the formation of hydrogen bonds and to the van der Waals' forces between the fiber surface and the dye. Moreover, the surface dye adsorption vs. the total dye adsorption curve had a same linear relationship independent on the molecular structure of dyes. The surface area covered by the adsorbed dye molecules, which was calculated from the reciprocal of the slope of this straight line, 5.26×106cm2/g fiber.