The influence of thermal treatment of olive oil and sunflower oil in the presence of air on their antioxidant activity was studied. Two parameters were varied: time and temperature of thermal treatment. The parameters that correspond to the real parameters of food preparation were selected, namely the temperature of 150 ° C and 180 ° C and the time of 10 min and 20 min. Antioxidant activity was monitored by measuring the relative concentration of 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical by electron spin resonance (ESR) during 20 min after the addition of oil to the radical solution. Four samples of olive oil and one sample of sunflower oil were investigated. It was shown that for all samples the heat treatment, regardless of the time and temperature of treatment, leads to an increase in antioxidant power against the DPPH radical. This can be explained by the presence of radicals formed during the heat treatment process as a consequence of lipid peroxidation. These radicals probably contribute to a faster reduction of DPPH. The different antioxidant activity of thermally treated samples most likely originates from the different chemical composition of the oil samples. Some samples showed better activity during treatment for 10 min at 150 ° C than when treated during the same time interval at higher temperature, while others showed stronger antioxidant activity after treatment for 10 min at 180 ° C. For a longer treatment time (20 min) at 180 ° C, antioxidant power decreased for most samples, compared to a shorter time treatment or treatment at a lower temperature. In sunflower oil, tocopherols are responsible for its antioxidant activity. Their role in neutralizing newly formed radicals appears to be different from the role of polyphenols in olive oil. It can be assumed that tocopherols react less well with newly formed radicals than polyphenols and therefore the role of these radicals in neutralizing DPPH is significantly more pronounced in sunflower oil compared to olive oil