Abstract: Abstract: Using a small amount of methacrylic acid (MAA) and hydroxyethyl methacrylate (HEMA) as functional monomers (total mass fraction less than 6.0%) in styrene butyl acrylate emulsion copolymerization, the viscoelastic behavior of the copolymer and the mechanism of change were studied by means of DSC、DMA and rheology. Results showed that the glass transition temperature, the storage modulus, viscosity of the polymer increased with the increasing of dose of functional monomers, and the viscoelastic behavior changed significantly when m(MAA)∶m(HEMA) are 0.5∶4.1 and 1.2∶2.5. GPC tests showed that the change of the viscoelastic behavior was not caused mainly by the molecular weight and distribution. Gelling was found by dissolving copolymers with this two proportions in ethyl acetate. FT IR results demonstrated that the gelling was originated from different types of hydrogen bonding, and was also the main reason leading to the viscoelastic behavior change.