The industry of polymers has a tendency to recycle and reuse as much waste material as possible. This raises the question, does the recycling process affect the thermic properties of the new polymer products? The main goal of the thesis was to provide an answer to the above-mentioned issue. The research was focused on the thermal properties of high density polyethylene (HDPE) because it is one of the most used polymers in the plastics production process. To define the thermal properties of HDPE, the temperatures of the melting and crystallization points had to be determined, as well as the enthalpy and the level of the crystallization. For this investigation, samples of a non-recycled HDPE and samples that were recycled 50 and 100 times were used. Polymer mixtures were prepared by mixing non-recycled HDPE with the recycled polymer at mixture ratios of 10/90, 50/50 and 90/10. The melting point was determined to be in the range of 131,4 °C to 138,6 °C, while the temperature of crystallization was between 113,8 °C to 118,9 °C. The enthalpy of crystallization decreased with an increasing amount of recycled material in the mixture. A drop in the level of crystallization was also determined. There was a 14,44% drop, from the 10% recycled material mixture to the 90% for the 50 times recycled HDPE compared to the non-recycled HDPE. For the 100 times recycled HDPE, there was an 18,57% drop from the 10% to 90% mixtures in comparison to the non-recycled HDPE.