Emulsion copolymerization of acrylic acid and vinyl acetate has been carried out and the product has been applied to the warp sizing of hard twisted fine colon yarn for voile fabric. It has been revealed that the sizing effect is remarkably increased when the hydrophobic copolymer is added to the sizing formulation, consisting of polyester resin, modified starch and Poly aryliny alcohol owing to the reduction of yarn entanglement in the saw box. And the entanglement is dependent on the dean of yarn swelling in size solution. These led up to a conclusion that the improvement of sizing efficiency is related with the hydrophobicity of the copolymer.