The wall-slip phenomenon during extrusion of polymer fluids is an important characteristic of viscoelasticity. The wall-slip behavior and the factors affecting it for natural rubber (NR)/cis-polybutadiene rubber (BR) filled with carbon black compound were investigated by using a Monsanto processability tester in a temperature ran e of 90-130degreesC and at shear rates that varied from 50 to 10 s(-1). Under simplified supposition conditions, a mathematical model for description of the relationship between pressure variation, polymeric properties, and extrusion operation parameters when wall-slip phenomenon occurs was established by using a tensor analysis method. On the basis of this model, an equation for estimation of critical slip distance (or position) was derived. The results showed that the measured critical pressure drop, when wall-slip begins to occur, was consistent with the theoretical predictions. The wall-slip phenomenon of the sample happened near the exit of the die. (C) 2004 Wiley Periodicals, Inc.