Thermal effects occurring under tensile deformation were analyzed for polyethylene samples with different level of crystallinity and dimensions. Temperature changes were measured by IR camera at all stages of deformation (from elastic region until fracture) at various strain rates. At the initial stage of drawing energy is consumed and temperature of the polymer slightly decreases (endothermic effect). Onset of necking gives rise to intensive heat generation and substantial increase in temperature in the element where neck originates. When front of neck moves away, analyzed element begins to cool down. At the region of strain hardening a new increase in temperature is observed. Maximal degree of heating during necking was found to depend on the level of crystallinity of the polymer, tensile rate and the speed of the front of neck. Calculations were conducted to estimate the maximum adiabatic temperature provided no energy is dissipated outside the heat source. Adiabatic temperature was compared with values of temperature registered in the experiment for specimens with different dimensions. It was found that starting from strain rate 500%/min deformation process of rather "large" samples proceeds nearly adiabatically. (C) 2014 Elsevier Ltd. All rights reserved.