The elongation behavior of various polymer melts extruded from the single screw extruder has been studied. Low‐density polyethylene, high‐density polyethylene, polypropylene, and their short glass‐fiber (GF)‐filled composites were used. It was found that the spinning stability was good but the drawability was poor for GF‐filled polymers. The elongation viscosity was measured by use of a Gottfert tensiometer (Rheotens) in the strain rate range from 0.1 to 4 s−1. In regard to the viscosity calculations, a very convenient and straightforward method, especially for molten materials without die swell, is presented. All the sample melts showed slight tension thinning; that is, the elongation viscosity decreased along with the strain rate. It was also found that the elongation viscosity of the GF‐filled polymer may increase along with the fiber content up to a critical value, and then drop if more fiber is added. The influence of the die geometry on the viscosity measurement is discussed. © 1997 John Wiley & Sons, Inc. Adv in Polym Techn 16: 199–207, 1997