An amphiphilic acrylic copolymer/montmorillonite nanocomposite was synthesized by way of a two-stage radical copolymerization of emulsion intercalation and solution intercalation, using acrylates, methacrylic acid, and acrylamide as monomers. The water-based product was designed with a structure of a hydrophobic copolymer dispersed in an aqueous copolymer solution to meet the needs of warp sizing agents on adhesiveness to hydrophobic and/or hydrophilic fibers, appropriate glass transition temperature, as well as water-solubility. XRD patterns and TEM observation for the product films showed an exfoliated structure of montmorillonite (MMT) in the part of soluble acrylic copolymer as the MMT content equal to or less than 7%, while in the part of emulsion copolymer alone or the product as a whole, the MMT showed an intercalated structure. Determination of DSC indicated that chemical bondings occurred between the hydrophobic and hydrophilic components, which guaranteed storage stability of the system and water-solubility of the sizing film, and that there were three T(g)s corresponding to the emulsion copolymer, solution copolymer and the interfacial copolymer between the two phases. All of the T(g)s increased with increasing content of the MMT in the composites. The elongation at break of the composite films increased with increasing MMT contents up to 3 wt % and then dropped down, and the tensile strength and abrasive resistance of the acrylic copolymer increased with content of the MMT.