The rubber particles included in rubber modified polymeric materials such as acrylonitrile-butadiene-styrene (ABS) polymer and impact modified polymers play an important role in determining their rheological properties, processing behavior, and mechanical properties. In this study both small strain oscillatory shear viscosity in the frequency range from 10(-2) to 10(2) s(-1) and uniaxial elongational viscosity behavior at two elongation rates (epsilon = 0.1 and 1.0 s(-1)) over the range of temperatures from 140degreesC to 200degreesC were measured for commercial ABS polymers with different contents and deformability of rubber particles. The influences of rubber content and deformability on rheological properties such as melt elasticity, elongational viscosity, strain hardening and/or softening, the onset of nonuniform deformation, and thermoforming performance were investigated. The Wagner two-parameter nonlinear viscoelastic constitutive model was used to describe strain hardening behavior, while the Considere criterion was used to determine the onset point of nonuniform deformation. The part thickness distribution obtained through use of a vacuum snap-back forming process was simulated to investigate the effects of rheological changes associated with different rubber particles on the thermoforming performance. It was found that ABS polymers with larger contents of hard rubber particles exhibited more melt elasticity, stronger strain hardening, a maximum of biaxial elongational viscosity, onset of nonuniform deformation at later time, and better thermoforming performance. Strain hardening and the Considere criterion provide simple, reliable Indicators of the thermoforming performance of ABS polymers.