Abstract: The VAm size was modified through adding the third monomers that were copolymerized with vinyl acetate-acryamide(VAc-AM) binary system.The monomers evaluated included acrylic acid(AA),methyl acrylate(MA) and acrylonitrile(AN).The influences of the structural units formed by above monomers on the performance of the VAm size was investigated in terms of the viscosity,adhesion-to-fibers,and film behaviors of the sizes.The result demonstrates that the significant improvement of the performance of the VAm size has been achieved by adding the third monomers.The adhesion of the VAm size to cotton fibers is strengthened by introducing AA units;the adhesion to polyester fibers is enhanced through incorporating MA units;and the adhesion to both polyester and cotton fibers is increased by means of introducing AN units.Incorporation of AN units into the VAm size is more beneficial to raising the working performance of the size.