Transient extensional viscosity of low-density polyethylene was measured by Sentmanat extensional rheometer in combination with MCR301 rheometer (Anton Paar) at different temperatures. Issues related to the experimental procedure, namely fixing the sample and controlling the temperature, as well as correction for true sample dimensions in calculation of extensional viscosity of polymer melts, were discussed. The molecular stress function model was used to describe the experimental data. The results were in accordance with other test methods and theoretical description when the measurements were done without using the sample fixing clamps, careful temperature control was followed, and the experimental data were corrected for sample dimensions affected by thermal expansion and pre-stretching at the beginning of the test.