The diploma thesis deals with the characterization and evaluation of a significant group of polyolefin plastics, with the focus on the two most widespread representatives: polyethylene and polypropylene. Because of their widespread use today, it is important to know their behavior under the influence of high temperatures. The main objective of the thesis was to evaluate the flammability of polyethylene and polypropylene based on STN ISO 1210 (1996), with focus onease of ignition, drainage time, burning time, weight loss, linear and mass burning rate. We tested two sets of samples from PE and PP, while one set was conditioned for 48 hours at 23 ° C and 50% relative humidity - reference samples and the another one set was conditioned for 168 hours at 70 ° C, which with regards to the norm meant aging of plastics. Sample testing was carried out using Method A, according to which the samples were tested in a horizontal position and Method B, in a vertical position. Based on the tests, the reference samples PP were classified according to the standard using method A into the FH-2 class and the reference samples PE altogether with the PE and PP after aging into the FH-3 class. Using Method B, the reference samples PP were included in the FV-2 class, samples PP after aging, the reference samples PE altogether with samples PE after aging were not classifiable on the basis of Method B, because their values fall into Class 4 and their sample value is with the same conditioning according to method A, therefore we classify them into class FH-3.We came to the conclusion that the aging of the material as well as the position of the sample in which it is initiated by the source of ignition (vertical or horizontal), affects the evaluated parameters of polyethylene and polypropylene. Post-Aged samples PP while using both Methods A and B fired-through the 100 mm marks, while the reference samples PP failed to fire-through in any method. The effect of aging is also monitored in PE, where after aging samples had a lower burning time with the same fire-through length, higher weight loss, linear and mass burning rate in comparison with the reference samples.